You might be deyhrated drink more water I was told by my ob and the er doctor that when you're deyhrated your muscles tighten and cramp.
Sounds like round ligament stretching. Same happened to me.
Could be round ligament pain as well
Can I really get that at 16 weeks
Constipation or Braxton hicks... It's tough to say. As long as your not bleeding with those cramps you should be fine. Try putting a heat-pad on your back and relaxing.