What Causes Brown Vaginal Discharge With Stomach Cramps And Headache?

i am a 18 year old female. i have been taking the pill but not every day as i often forget, but have still been having sex with my partner without a condom. my last period was on the 12th of feb and within the last week or 2 i have been experiencing a light brown discharge with stomach cramps & the odd headache & sometimes feeling sick. i was wondering if you could tell me what is wrong? i've never had this brown discharge before