Pinching at Uterus. Is It Normal?

Hi. I just turned 6 weeks today. And for the first time this morning, I did a little exercise I just saw on youtube (for 1st trimester exercise). On the middle of the exercise, I felt some pinching in my left uterus. Naturally I stopped exercising. Even this afternoon I can still feel some pinching. Hindi naman frequent but it makes me worried. Is that the fetus reacting or what? What causes the pinching? Is my little sweet pea safe in there?