What Are The Chances Of Pregnancy When On Birth Control Pills?

Hi. I am taking Tri Sprintec birth control pills, but I haven t yet had sex per-say. Last month my period was lighter than usual but I thought nothing of it. For the last 5 days I have had diarrhea and been very slightly sick to my stomach at times. I m wondering if I could be pregnant. I haven t had real intercourse but there could have been a chance that some sperm got on my vagina on the outside and then somehow moved in and fertilized. I thought the diarrhea was from the rich foods that I had been eating for the last few days, or maybe a bug that has been going around at work. What should I do?