At 21 weeks, some lower abdominal discomfort can come from round ligament pain or the uterus stretching, which is pretty common, but period-like cramping is something worth taking more seriously since it can sometimes signal something that needs attention. I would say it is hard to tell just from how it feels whether it is harmless or not, so the safest move is to get it checked by your gynae or go to KKH rather than wait and see. It is always better to have a doctor rule out anything concerning, just to give yourself peace of mind.