It sounds like the onset of arthritis.
Do you have a heating pad or electric blanket?
If so, plug it in, turn it on and put your dog on the pad or blanket. If you are using a pad, be sure the heat is in contact with the affected area of your dog’s body.
This will not cure your dog but it is theraputic and your dog will feel better.

First off,allow me to emphasize that I am not a vet,and I strongly recommend that you get your vets ok before adding any of the supplements I will suggest to your dogs current regime.That said,don’t be surprised if your vet says they don’t see the possible benefits of these so called "alternative" practices,most allopathic vets don’t.Even my vet who has seen the results multiple times with my dogs still seems to think the dogs cured themselves coincidentally.(which is fine by me because she is still a wonderful vet.)Just make sure your vet says these things wont cause any harm,or affect any meeds your boxer may be on.I have been working with dogs,esp American Bulldogs for 12+ years,and have had several dogs who benifited from the products I will recommend at therapeutic doses.I also include maintenance doses of these supplements in all of my dogs food.So you can see the effectiveness of these substances I will give several examples.My female A.B. suffered a subluxing patella and ligament damage,compounded by reoccurring lyme.After years of prescribed painkillers and crate rest/leash only walks,etc.My vet recommended surgury.I held off,due to unsatisfactory results several friends had with the operation,which left their dogs worse off than they had been before,and her weakened immune system.My 120 lb. male A.B. tore his ACL while defending our home from invaders,and was severely crippled and in excruciating pain.He was unable to walk much,had difficulty lifting his leg or squatting to relieve himself,and couldn’t even get up on the couch or bed w/o assistance.He was in such pain and despair of his condition that he lost a lot of weight and became very depressed.Due to his size and personality(he is protection trained,and loves only me and a select few humans)plus my general doubt about the results,I opted to wait until I had exhausted all other options before considering surgery.Last,but not least,my little puppy Clover was playing with a friends dogs and they accidentally broke the head of his femur.The surgery for this type of injury is to saw off the entire femur head and just wait and see how things work out.As I already was supplementing him at the time,he only showed symptoms on and off,and by the time I decided there was a major issue and got him x-rayed,my vet said his body was actually doing what they would have done by surgery.But he was in pain,running and wrestling one day,then crippled the next.He also walked/ran with a distinctly crooked gate.All three of these dogs recovered and are currently back to their old activities and pain free.My puppy totally straightened out and you cant even tell he was once so crippled.The products I use are MSM,Vitamin C,fish oil,and cayenne pepper.MSM is a sulphur compound which acts as a natural painkiller and antinflamitory,as well as helping to rebuild damaged tissues and connective tissue.It also has a myriad of other benefits.You can most likely find it in your local health food store or GNC.I prefer the crystal/powder form to the capsules or tablets(which I haven’t found nearly as effective.)This product is so amazing I even began taking it myself for a connective tissue disorder I was born with,and I can def. testify to its pain reducing effects.MSM works best when taken w/ vitamin C.Start off with a tiny dose of each,and gradually increase either to bowel tolerance or to the dose recommended by your vet.Vit C alone is also a pain reducer and antinflamitory,but the combination is most effective.Once again,I prefer the powdered vit C.If you want to do even more,add in a good quality fish oil and some cayenne pepper.With the cayenne,its also best to start out slow and gradually increase.With all of these supplements,make sure to get the best possible human quality products that you can afford/find.Im not sure what you feed your dog,so you may have to experiment a bit till you find a way to get all this good stuff into her system.I feed an all raw homemade diet,so I have no trouble sneaking these things in.You might try(with your vets blessing of course)one or a combination of the following:raw(or cooked)hamburger,raw eggs blended w/ a bit of water,yogurt,cottage cheese,applesauce,etc.You could even blend the supplements into peanut butter and make a mini sandwich for a picky eater.MSM is bitter,so thats likely the one pets may notice,Finally,I will mention a product that I have used in the past and would not hesitate to use again if needed.It is called Prevention,and is available through Joshua Kennels American Bulldogs website.The testimonials are quite impressive,and i can vouch for the product as well.Good Luck,hope this helps!