Hamsters, gerbils, mice and rats are bred so frequently and at random that congenital defects are very common. On the occasions when siblings are allowed to mate (especially after repeated generations) size of litter will decrease, and less of the young will survive. Of those remaining many will have physical deformities.

Unfortunately those that you can see are sometimes the tip of the iceberg. In other words, sometimes signs of a physical deformity on the outside mean much more deformity internally, leading to complications in digestion, reproduction and more, and eventually to decreased lifespan. I am not saying that this is what is wrong with your hamster but it is something that you need to be aware of. In all cases when an animal that you have decided to be responsible for becomes unwell, you need to seek proper medical advice. Please ask your mum to take you and your hamster to the vet for his opinion.