AF is a month late

I took a hpt 4 days after AF was supposed to come around and got a bfp, but the three I've taken since then (including one yesterday) have all been negative.

I haven't been under any unusual amount of stress, my diet/exercise hasn't changed, nothing is out of the norm except for the fact that I am late.

I have noticed that I've been really tired, cry at the drop of a hat, and everything seems to make me nauseous lately, with lower back pains and headaches (but not the ones I normally get with AF).

I don't know what to do.. Do I wait a couple more weeks for AF? Or do I make an appointment with my doctor and see what's going on?