Spotting -6 weeks

I just have some questions about spotting. I went to the bathroom a bit ago and there was some light brown drainage on my panties(nickel sized). Then when I wiped there was one pea sized brown spot. That was it, no more. I am 6 weeks and have had no spotting at all the whole time. Anyone had this happen? Should I be alarmed? I had sex around noon today, could that have caused it? Of course I will call the doc in the am, but until then I wanted some piece of mind. Thanks.