According to web md:
"Round ligament pain refers to a type of pelvic pain caused by stretching of the round ligaments. This occurs more commonly on the right side of the pelvis...The ligaments pull and tug on nearby nerve fibers and sensitive structures, causing pain. The severity of pain can worry you. Although round ligament pain is uncomfortable, it is also very normal. A ligament spasm, an involuntary contraction or cramp, usually triggers a sharp pain. These spasms are found more frequently on the right side than the left because of the normal tendency of the uterus to turn to the right."
If it's just once in a while, it could be round ligament pain (which can be anything from a dull ache to sharp pain)....but if it's more of a constant pain, I would have your doctor check it out.