I have a lump in my right armpit and it really hurts what is it ?

Abscess: The tender lump in your armpit is likely a staph abscess. You need warm soaks to help it come to a head and drain and you probably will need antibiotics. If it drains from the compresses, you may be able to avoid antibiotics.

Infection: It could be an infected hair follicle or an infected lymphnode, You ought to see a doctor for proper treatment so that you do not end up chronic hideradenitis suppurativa.