What Causes Sudden Brown Discharge And Discomfort In The Stomach?

Heya, within the space of a week the last few times my husband and I have been having sex, the following day a brown discharge will come out. My tummy has been uneasy. I am due for my period next week, I am not on birth control. I've been thinking about going for a PAP smear after my period as it has been almost 2 years since my last one. Worried... any possible answers?