Hi-I also had a lump or enlarged lymph node under my right armpit. My lump appeared after gall bladder surgery. I developed fibromyalgia 6 months after my surgery. I told the doctor that the fibromyalgia flares during my cycle. I believe the lump and the fibromyalgia is hormone driven. I recommend anyone who discovers a lump like this to have saliva testing done on your hormones. The hormones they test for are: DHEA, testosterone, estrogen, progesterone and cortisol. My hormones were not balanced and they were low. I am 43 and this happens to women in premenopause. You can replace your hormones to balance them through supplementation or creams. What helped get my lump pain under control was a diet with no sugar(except for honey) low to no fat (except omega 3fat and extra virgin olive oil)no white flour, and no caffeine. I eat protein with almost every meal to help with energy, complex carbs, lots of raw veggies, fruit and water. I also take a multivitamin, extra vitamin c, extra vitamin e, 2000 units vitamin D, calcium 600 and magnesium 300mg two times daily, last but not least digestive enzymes with my meals. You diet, srtess and age will cause your hormones to become out of balance. As of now my painful lymph node has gone away.
It is hard at first to follow this diet but once you get past a few weeks it gets easier.
Good Luck everyone.