BLeeding day after sex

I have a question i had sex on march 16th after not having sex since november 14th 2004 so you know its been a while well today which is the 18th of march i started bleeding its just like spotting and its not time for my period i was spotting yesterday also and i just thought it would stop but it hasnt and its not like a period beings its not time for my period i just had it like 2 1/2 weeks ago so i was wondering what it could be if anyone can help me please thank you:)