I am so sorry to hear your hamster has passed away. It is a very hard time. As Souffle says, you have done nothing wrong. It was her time by the sound of it and it also sounds like she had something going wrong inside, whether it was related to the lump in her mouth or not. I agree it sounds like she was dying and in her final stages - and she was lucky to be able to do that in her familiar home with you caring for her. So very sad. Our last Syrian passed away about this time last year and it really is quite a trauma when they are gone, and it is a normal reaction to feel guilty and wonder what you could have done differently.
She is playing free over the rainbow bridge now, free from pain. Have a look at these two threads - there are some links to information about pet bereavement that I think you will find helpful, becuase they helped me.
Pet loss - where to find help, tips and comfort on the loss of a pet.
The Rainbow Bridge