What Causes Vaginal Spotting With No Regular Period?

I am on Minastrin Fe 24 birth control and my period was expected to come on the 26th of August. It is now the 29th of August; I last had unprotected sex August 8th. I never miss a pill, and I have been having light spotting along with sharp pains in my lower right abdomen. Is there a complication with me, am I pregnant, or am I just overreacting?