A week ago I noticed a lump under my armpit, it was going away, however, about two days ago I got a cold and it came back. What could it be?

Explanation . The first episode of lump under armpit , that going away by itself, may caused by intensive shavings, what may happen. Increasing in size with cold is possible as well and can be benign/innocent. But if will not go away with clearing "cold" you need to see a doctor .