That exact thing happened to me about 3-4 weeks ago. I had pain similiar to it before but that time it also spread to rectal area and I honestly thought I was going into labor, I couldn't even stand up! But after about 2 hours it just went away and I haven't felt it since. It's always good to call your doctor and get advice. My doctor always sends me to the hospital when ever anything is wrong and they checked to make sure I wasn't at risk for pre term labor, and of course I wasn't! They told me its round ligament pain. Basically as long as there's no blood or fluid you're okay.
I've gotten that..is stretching. It hurts like hell but it hours away,, or atleast you get used to it