I had a lump under my armpit but it's going away. what could it have been?

Different options: That area has a lot of things in it both in the skin and below it. Sweat glands, hair follicles and skin cysts as well as lymph nodes, fatty tissue, vessels, tendons and other things. Is the lump growing, inflamed, painful, painless, draining, recurrent or constant, moveable or fixed...? Each of these (and many more) will provide your doc the answers as to what it is and what needs to be done.