Opinions, please.

My husband and I have decided that we are ready to try for our first, we have been ttc for two months now. Lately I have been getting weird periods, for example: my period started on Sep 2 and was very light and lasted for 3 days, I normally have them about 5 days and they tend to be medium or heavy, and my most recent "period" which started on the 29th of September and ended the next day (30th) both days were seriously nothing but light spotting, it was brown in color and I have been having light tan blood when I wipe every now and then since my short lived period. I have taken 4 pregnancy tests between this time, and all came out negative. My annual pap is scheduled for the 19th of this month so I am hoping that I can talk to my doctor about it and see what she thinks. I have been hearing stories like mine, that ladies experience what I have then find out they are actually pregnant, do you think it's possible that I could be?