My Cat has a Lump on His back?

My Cat Sammy has got a lump on the middle of His back. It makes his Fur poke out. I fear it may of came from another Cat, because about 4 days ago he accidentally escaped through the back door. And He didn't return until 2 1/2 days later. I've searched on the Internet what could it be, but there are so many Viruses, Infections, etc. that the Net listed and I'm confused on what it is. It doesn't hurt him when you push the lump lightly. He has a normal appetite, normal urination/Feces(except the day he came home, he had diarrhea, but that was probably because I fed him a Moist Cat Food that had Tuna and Egg in it.)