I had some spotting around 15 weeks, but it didn't last long and there wasn't a lot. The doctor told me not to worry unless I was bleeding a lot (like a period) or if there was cramping along with the spotting. He found the heartbeat during that appointment and put my mind at ease.
Not to stress you out but you should call your doctor! Any concers I have I always call! Congrats and hope your appt goes well!