^^^^
Sounds like a sebaceous cyst - is the puss like a mix of white, grey, yellow and maybe a bit of blood and really foul smelling?
They are not unusual and has nothing to do specificially with AS use - anyone with skin can get one - hell people can have them under the skin in various parts of the body and don't know it. A lot of the time it's some sort of trauma to the skin or an infected hair folicle that will bring them to be noticed - tender and possibly painful.
If it a SC - the sac under the skin has ruptured and that was the fluid is and being that the lump has grown it would be a very good idea to see a doctor - it you can get to the dermatologist go there first - if not try and get a referal.
You'll be fine - just get it checked/fixed - they'll either remove it or treated with steriods/antibiotic or both.