brown discharge?

I'm on the pill, I take it at the same time everyday for 2 years. I just took my last pill last night. I'm due for my period on Tuesday but the last two days I have had brown discharge, period pains and sore boobs. I haven't had sex for 4 months and pregnancy tests have been negative. 
Is there a reason why I am having brown thick discharge? Any advise would be appreciated!