Yes as carol said it could be a fatty lump..take her to the vets..they may well tell you to keep an eye on how fast its growing..if its growing fast then i don't wanna frighten you but thats when you have got to worry.
My mums dogs have fatty lumps and they are old.
My old girl had 2 lumps at different times that grew fast(ish) and they turned out to be cancer she had ops and she's still here today to tell the tale