Severe PMS with IBS-D, diarrhea, nausea.

Hello doctor, I have IBS-D and I notice a very regular pattern linked with my menstrual cycle. About 10–12 days before my period starts, my PMS symptoms begin — I get nausea and frequent loose motions (3–4 times daily). This continues until my period starts. During my period, I again experience diarrhea, nausea, and stomach discomfort. Even after my period ends, the symptoms don’t completely settle — for a few more days I still feel nausea and loose motions. Another problem is with my appetite. A day before my period, my appetite increases a lot, but on the first day it suddenly drops. For the next 5–6 days it stays average. From around the 7th day after my period, I completely lose my appetite.. I don’t feel like eating at all. Even if I try, after just 2–3 bites I feel full. Because of this cycle, I am unable to gain weight and in fact, I am losing weight. This is causing me a lot of stress. Could you please guide me on this?