I have the same thing happen when I sneeze. I will sneeze and it is almost like I get a period cramp. I can feel my whole uterus tighten. It is crazy. I would not worry about it though. It is a bit uncomfortable, but I am 28 weeks pregnant and the baby is doing great.
===> I found it easier to bend over slightly while I sneezed or coughed.
Mmm... But how about when I lie in bed??? I do not know if I have ever pulled a stomach muscle before.
Have you ever pulled a stomach muscle before? And then coughed or sneezed? It's slightly similar to that. Your core (or torso) is very involved in almost everything that you do, so when you cough or sneeze or even hiccough hard enough, you stomach muscles tense and react to the activity. It's probably going to feel like that for a bit, until you get used to it. I sneeze a lot, so in ths beginning I found it easier to bend over slightly while I sneezed or coughed.