What Causes Delay In Periods,brownish Bleeding,tiredness And Sore Lower Back?

I think i might be almost three weeks pregnant but i am not sure, my period is one week late, but now in the afternoon i started to have Brown-ish light pink blood, though yet i am still experiencing tiredness sore lower back,
I'am not sure if my period was late because some months i can get it on the first tuesday of the second month. so i am really confussed.
any help would be great.