May be due to gastritis or peptic ulcer... Consult nearest gastroenterologist for endoscopic examination they will do the needful... With that try eating several small meals instead of three large ones whenever you can...Avoid foods that irritate your stomach, especially those that are spicy, acidic, fried or fatty... avoid coffee,tea etc... Avoid stress,consider calming activities, such as meditation, yoga etc...sleep with head raised position and have dinner at least 2 hours before sleep..good luck.

Please  read my blog on PCOS. Hope that helps.

Periods are KNOWN to be irregular.......at 21 your hormonal pattern is sometimes not set and this can cause irregularity due to stress, emotional disturbance, and some medicines....... Consult a Gynecologist for the periods as it needs an blood tests and examination to check your internal organs....... Regarding the pain, it is mostly due to gastritis or indigestion.......NEVER sleep down after a meals within 2 hours, as your stomach is full and if you sleep the contents try to come up........ You may also get a surgeons opinion or gastroenterologist's opinion about your pain