Hi N.,
Unfortunately, it's impossible to tell really. You are either getting your period, or it could be implantation. Although, I think actual spotting from implantation is fairly rare. Usually it's just a tad bit of cramping. If you are tracking your temps, that can really help take the guess work out. If your temp drops and you start to spot, then your period is on it's way. 6 months is not too long to worry about. I'm not sure how old you are, but for women over 30, it takes an average of 6-8 months of deliberate intercourse to get pregnant, and most doctors don't even want to discuss it unless it's been a year. If it were me, I'd start tracking my temperatures, that way you can really see when you are ovulating and where your temps are. If you feel like something is wrong, go to your doc and check it out. I did. My cycles started to change dramatically, and get really short. I'd start spotting for a week before my period and spot for a while after. I went to see my doctor and turned out I had hormone issues, and needed some help. So, listen to your body and keep trying! :)