I have a lump that has appeared just today under my armpit and it hurts really bad. it is still not 100% visible but it does hurt when i move my arm or touch it. what should i do and what could it be?

Maybe infected: Lumps in the armpit skin of a teen or adult can be due to ingrown hairs or blocked oil (sebaceous) glands. Occasionally, they are blocked pores, filling up with dead skin cells. If a lump hurts, it may be infected and would need antibiotics or need to be cut open by a doctor, depending on how it looks. Lumps deeper in the armpit may be enlarged lymph nodes, which can be infection or rarely cancer.