Strange tightening sensation in stomach

I will be 16 weeks tomorrow. A few days ago I felt for the first time this weird tightening feeling in my stomach getting up off the floor from stretching. On top of the tightening, my ab muscles felt like I had been doing crunches (but of course I hadnt ). It's happened again yesterday after I finished a prenatal exercise video and then again this morning for no apparent reason. It's not painful, just uncomfortable, like its involuntary and I can't relax my stomach when it happens and my belly feels very hard. Starting to scare me...has anyone else had this and know what could be going on? I plan on calling my midwife Monday if it continues, but I'd hate to call now unless it's an emergency.