I don't mean to sound ugly but if you don't want to get pg then why are you having unprotected sex?! Not very smart, if you ask me. Sorry but this sounds like MUD. Just someone trying to get a rise out of women who are really wanting and trying to get pg.
I guess it is a possibility, but I don't really think you've had a miscarriage. Like your doctor said, it's probably from the Plan B. The hormones could very well have messed up your cycle and cause all the bleeding/spotting. But why are you not using protection if you're worried about getting pregnant?