What Causes Severe Pain Under Armpits?

Hi, I have been experiencing severe pain on both sides, just under my armpits for a few months. The pain only happens in conjuction with the start of my period and lasts a few days. I have never experienced this before. I m 39 and contracted Pnumonia in both lungs in January which despite repeated antibiotics has yet to shift. could This pain (which was before illness) be an indication of weakend immune system ? If so, what sort of illness might be causing it? Thanks so much