Woke up with sudden armpit lump. Very small beneath skin. Pain to touch. No redness. A bit raised skin. I scratched abit. No family history of cancer. Cause?

Folliculitis. This sound like it could be inflammation of hair follicle or other small irritation. Watch it and if still present after month or one menstrual cycle have it looked at. Sounds benign at this juncture but hard to say for certain without examining it.