What Causes Spotting With No Regular Period After Missing Birth Control Pills?

Hi I am a twenty one year old female who is 5'5 and 208 lb. I am wondering if I may be pregnant. I am currently on the "pill" but I missed a few pills and had sex without using a condom. The next day I resumed my missed but my period did not come on I only spotted. I am worried because I have missed pills before and did the same thing but my period came on and was regular. I took a pt and the results were negative.