Tummy Tightness?

It's too late too call my doctor and I've already called the ER. I'm almost 16w and have been feeling uncomfortable tightness in my belly. I've noticed it for a little bit but haven't felt I should be concerned (til now because I googled). I don't have any bleeding and sometimes I cramp when the tightness occurs. Has anyone gone through this and does it sound like I should be concerned? This is baby number three but I don't remember going through tummy tightness til last Tri.