When should you go to the ER

I’m 22w3d and I have been having this sharp pain in my lower belly that shoots down my pelvic area. I’m thinking it’s the pregnancy, uterus growing. But my husband and I just had sex about 10 mins ago and now I’m cramping really bad, through my whole lower stomach. I never had this with my first two pregnancies so I don’t know what to think of it. I read to drink lots of water, could be from dehydration, so that’s what I’m doing. But when is a huge warning sign to go to the ER ? I really don’t want to risk anything but I don’t want to go and it be nothing.