don't worry every rat i have ever owned has gotten a lump the only problem was after a while it grows and may cause problems with regular day to day activities.... I'm glad you took her to the vet and am hoping you come back with good news... yes 2 is old for a rat and is why she probably had the lump it could just be a lump of fat as well don't get yourself all worked up until you know what it is... so it is very common especially in female rats... Is it female? You might have already stated this.
Good luck

here read this I found a link:
http://www.rmca.org/Articles/tumors.htm

Hope I helped