I think my rat has a tumor

Question
My pet rat all of a sudden has a hard marble size bump on his stomach, it is
right in the center. This bump appeared within a day and the only thing I can
think is that it might be a tumor or a hernia.

Answer
 A tumor or a hernia is a possibility, as well as an abscess.  Are you able to move the lump around or is it firmly attached?  Tumors are less common on the stomach in male rats but they can occur.  They tend to be more loose rather than attached, though if it is coming from something inside it may be more firm.  Another likely possibility is that it is an abscess, basically a large pimple.  They usually result from puncture wounds that heal over quickly but become infected, festering into a pus-filled bubble of skin.  Again it is more rare to have one on the stomach but definitely possible.  The best option for you is to take him to the vet.  The vet will be able to feel it and aspirate it (stick a needle into it) to tell what it is.  He or she will also be able to discuss treatment and costs involved to help you make a decision on what to do about it.

 Well I hope everything turns out, let me know what happens!  Good luck!