Looking for reassurance.

I started taking the Pill in August. I have been through 2 packs so far. Last week I had my period (or withdrawal bleed) and had sex on the last day of it (no condom, but he pulled out). 2-3 hours after sex I took my first pill of the new BC pack... and an hour or two after that I started having some spotting with minor pains.

I am now concerned of what it may mean. I REALLY do not want to be pregnant, hence the Pill. I know its very unlikely that I could be pregnant but I am still worried that something is not right!? Any insight at all would be helpful! Thank you!