Your rat may just have an abscess. It would, however, be in your rat's best interest to have it looked at (and removed) by a vet either way.

I should mention that an abscess lump will usually feel likes it's firmly attached to the skin, while a tumor usually feels like it's "behind" the skin. The following articles will provide more information:

Tumor Removal and Baytril:
<http://www.rmca.org/Articles/tumor.htm>
Tumors in Rats:
<http://www.rmca.org/Articles/tumors.htm>
Skin Abscesses in Small Rodents:
<http://www.rmca.org/Articles/abcess.htm>

These links will take you to our rat-friendly vet lists, sorted by state:
<http://www.rmca.org/Resources/vets.htm>
<http://www.ratfanclub.org/vetref.html>