Explaining cat in "heat" to children

My cat just recently is in heat and is howling and rolling around, we are going to spay her asap but in the meantime my 5 year old is asking what is wrong with Kitty..
   
What is good G, PG answer?

for a 5 year old I would just simply say that she is being a bit silly, or maybe if she is due for a spaying soon tell her that she isnt feeling very well but the vet will be able to help her soon and she will have an operation to make her feel better??? that way 2 birds with one stone so to speak