Am i 4 weeks pregnant?

okay, I had the depo shot in march. March 13 to be exact. I got it again in June and that was the last. I had unprotected a lot in the last month, but the first time would've been September 16. I don't get a period, though aug something like 24 i had a light breakthrough bleeding. I have been feeling very tired this past week, my brest get very tingly and sore when my fiance goes to touch them around the nipple area. Also around bed time they hurt near my arm pit area, and i get random headaches. After , i get kind of nauseous, and when im ringing out certain foods at work, it happens. I haven't gotten sick yet, but im not sure whats going on. On my calculations, if im correct, id be about 4 months pregnant... Help?