As the others said, it could be any number of things, it depends on where the pains are, how the present and your activity at the time.
If it concerns you, you should speak with your obgyn.
It could also be the ligaments that hold up your uterus stretching. Got that alot with both mine, painful but not serious. Talk to your doc if it continuse or gets worse. GL to you and your angel.
Quite a few things. Contractions, the baby, muscles stretching. If it's anything that worries you or is unbearable, you should always talk to your doctor.