Suggest Medication For Cramps,dark Colour Discharge And Spotting

hi, I have been off my birth control pill for about 2 weeks( since my last period), because the new pack got lost in the mail and I cannot access it from the cvs in my city. I had sex (Im pretty sure we used a condom but I was pretty drunk and can t be positive) on saturday, and now have a dark color (seems darker than period blood) found in my underwear. Im not sure if I am spotting or starting my period early or what (it kind of feels like when I start my period, but usually I have more cramps beforehand, and usually start on a wednesday. Is there any other reasoning for the dark spotting (pregnancy, std, ext?) I am considering buying plan B if there is a chance of pregnancy.