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

[Sheena]

is that by the base of her tail? the picture isn't too clear.

with the location, it's a good idea to get her checked by the vet (sorry but this is the RESPONSIBILITY of a pet owner, and not an optional extra) but this is an inexpensive thing, rather than surgery.
get her a course of antibiotics.

it COULD be a tumour, but with it's location it could also quite easily be an abcess, which is a pocket of infection. antibiotics, if it's an abcess, can help to bring it to a head so that the infection can drain away. also, use warm compresses. get a cloth, and some hot water with salt dissolved in it, when it's cool enough not to be uncomfortable on your wrist, but still warm, dip the cloth in it, wring it out, and try and hold it in place over the lump for as long as you can. the heat can help the infection to come to the surface.
this may cause it to split open, but that is good. it allows the infection to drain out, relieving discomfort and allowing the infection to go away. keep the open wound clean, and gently remove scabbing with warm water so that it can continue to drain freely. the wound will gradually close up, hopefully with no infection left inside. the antibiotics will also help prevent reinfection.

of course it could be a tumour, but it's always worth treating for an abcess just in case.

at her age, if it is a tumour, the vet will almost certainly not recommend surgery. the risk from the anaesthetic would be far too high, and she'd be unlikely to make it. so if treating for an abcess has no results, it will be a case of monitoring how comfortable she is. if that is the location, this could quickly become problematic for her, pressing on her bowl and bladder and making it difficult for her to go to the toilet. so watch for pain or discomfort when she goes.
I expect that this will become a problem before her mobility is much effected by it.

when her quality of life becomes impacted then you will need to take her to the vet to be humanely put to sleep.
again, this is not an optional extra. the alternative isn't her passing peacefully in her sleep, but painfully from pressure on her internal organs, over a long period of time. so you will need to make that choice for her before she has no quality of life.