Hi and thank you so much for this query. Pain which is closely related with your menstrual cycle is most likely related to your sexual hormones and a tissue that responds to sexual hormones. In some women, there are accessory breast tissue in the armpit. This is very evident when they put to birth and realize that they have swellings in the armpit during breastfeeding. Just like breast become swollen and at times tender in the course of the menstrual cycle, so too can be this accessory mammary glands in the armpit. Because it is poorly located and has no space to accommodate expansive changes, it can be more disturbing than the breast. This is the top suspicion on my list. However, you will need to meet and get evaluated by your OBGYN. If this is the cause, then various treatment options would be presented to you. if it is also caused by something else, then it shall be identified and handled accordingly. Hope this helps and provides guidance on how to approach this problem. If you have more questions, please i a ready and very happy to answer them. I wish you get this figured out and the pains resolved. For now, you may want to try some pains relievers. Dr. Ditah, MD.