For a year and a half now my right armpit has swollen once a month. It stays like this for about 3 days. When it first happened I had an ultrasound done, and they saw nothing. Just went back and she confirmed the "lumps" were flared lymph nodes.

It is always painful to the touch, and more so when swollen. I have tried switching deodorants, stopping deodorant, stopping shaving, etc and nothing makes a difference.

What could cause this?