I get this all the time..
my midwife told me it was this..
Round ligament pain is generally a brief, sharp, stabbing pain or a longer-lasting, dull ache that you may feel on one or both sides of your lower abdomen or deep in your groin, usually starting in your second trimester. It happens when the ligaments that support your uterus in your pelvis stretch and thicken to accommodate and support its growing size. You may feel a short jabbing sensation if you suddenly change position, such as when you're getting up from a bed or chair or when you cough, roll over in bed, or get out of the bathtub. Or you may feel a dull ache after a particularly active day, if you've been walking a lot or doing some other physical activity. Call your caregiver if this discomfort continues even after you've rested.
Hope that is some help..if you are worried..call your midwife or doctor