Lumps on her bum

Question
I have a 2year old female dumbo rat, she has a large cage and i use a hypoallergenic bedding, i feed her on rat nuggets but also bit of veg and things. she seems to have a large  swelling on her rear and can't poop. I have taken her to the vet who says that she thinks it may be a tumor. She's on antibiotics at the moment. I was hoping that you could give some advice as to what else i can do to help her. I have been making her food softer so that it may be easyer to pass but i don't think it's helping. I have just had to put her sister down and i can't bear the thought that she may have to be put down also. HELP

Answer
Before I continue with advice, I wanted to ask, why didnt the vet suggest removing the tumor instead of allowing her to go without being able to have a bowel movement?
This can kill her if she becomes impacted. It isnt making much sense. Is she making any type of bowel movement at all and if not, when is the last time she went poop?