36 weeks and 3 days pregnant??? Is this a sign?

Yesterday I had what I think was a contraction. It felt like really bad menstrual cramps for a few minutes. It was only once but I was wondering if it is a sign that labor will start soon?? Just curious because I haven't ever felt a contraction like that yet. Usuallly my stomach just gets tight, but it doesn't hurt. Anyone know that has experienced this before???? When I went to the doctor at 35 weeks he said that I was starting to soften!!?!? My next appointment is on Thursday and we will have a sonogram. Just curious if this is early signs of labor?? Anxious!