What Could Be A Round, Brown, Raised Mole That Appeared Overnight?

My husband has a round, brown, raised mole that appeared overnight on the left side of his abdomen. He thought it was a bug bite because it burned and was slightly swollen and raised almost like a pimple. It is the size of this capital letter "O". It wasn't there yesterday. He's been in a lot of pain on that same area of his body due to kidney stones and having a stent placed last Wednesday and removed on Friday. He's been taking Lortabs, Flomax, Alleve and some ibuprofen, not necessarily all at the same time for the last 5-6 days. What could this be?