Although I believe in "never say never" I think it's pretty unlikely that you are pregnant. However, you'll only know for sure if you take a pregnancy test or start your next cycle. The spotting may be from something as minor as being a little too ummm, "rough", or it may also signal some other gynecological problem. It's best to go in and get checked so that you can cover all of your bases.

As far as the whole menstrual cycle, you have a greater chance of conceiving on or about the 14th day after the start of the mentrual cycle (based on the average 28 day cycle) since that is when the egg is released from the ovary. A couple of key things you need to remember is that an egg is viable and capable of being fertilized for 12 to 24 hours after its' release and sperm can live for up to 72 hours. Since I'm pretty regular and ovulate on about the 14th day of my cycle, I make sure that I am careful between days 11-17 from the start of my cycle. The one time my husband and I weren't we said "Welcome baby number three!" nine months later. Seriously, I can tell you the exact day I conceived her. Anyway, good luck and hope this helps!