Sudden large swelling

Question
My daughter-in-law's male rat has a large swelling on his hind leg.  She said it is as large as a chicken egg.  It has grown to this size in 2 days.  It is soft as if filled with fluid and starts at the leg, extending up toward the testicles, but not reaching the testicles.  Felix is 3 years old, very well cared for, bathed, fed, etc. and has been otherwise healthy.  He seems to have no pain.  Any ideas?  Thanks for being there...

Answer
Tumors dont grow out of the blue. An abscess will come out of nowhere and grow huge till it breaks open so it can heal.

From what you said, it sure sounds like an abscess. She needs to help it along by helping it burst unless she wants to have a vet do it, which will often end up with the vet lancing it and draining it and charing a fortune for something that usually happens on its own anyhow.  I wont say NOT to see a vet, but if its an abscess and not a tumor...well....
What she can do is hold a warm compress on it. Look to see if there is a black scab on it or a hole in it at all.  Prepare an area to use to work with and keep on hand a dish of water, some saline solution, neosporin, soft paper towels and a waste basket. Oh, mask to go over nose is  optional since these things can have a pretty hefty odor!
All she really probably needs to do is press it a bit and chances it will pop are high.  If not, after holding the warm compress on the lump, she can just put her fingers under the base of the lump and firmly but gently press in and up.   It should burst like crazy and will provide immediate relief to the rat if he was bothered by it!
Continue to press down till all the pus is out of it and irrigate it with some saline or else simply use plain warm water with a tiny bit of peroxide.  pat dry, fill up any hole with neosporin ointment.  It may fill up again, but if it was cleaned out good it probably will just close up and be gone in a few days. Continue to keep it clean and use the neosporin several times a day till it looks healed.