please help i found a lump on my rat!?!?

[WinniesMommy]

I was feeding my rat and she climbed up on her cage and I noticed a medium sized lump. I never noticed it before and I'm really freaking out now that it's a tumor. I looked it up and it said it's most likely a benign mammary tumor. But I'm just thinking of all the other things it could be and freaking out about it. Here's a picture, she was moving a lot so I couldn't get the best picture:

https://twitter.com/MyPetiteTweets1/stat...05/photo/1

What do you think it is? They didn't say how old she was when I got her but she stopped ovulating not too long after I got her and I've had her for over a year. So I'm guessing she's around 2 1/2 - 3 years old. My parents won't take her to the vet because they said it's too expensive and I'm only 14. Help?? I love her and I don't want anything to happen to her. Thanks x