Unexpected bleeding?

So, I've been on the pill since March, before that I had an IUD for 2 years. I'm not due to get my period for another 12 days, and I've been experiencing some bleeding, but it's dark brown. I've also been feeling pretty nauseous, with a low grade fever and a slight headache for the past two days, and I've noticed my breasts swelling/being tender. I took a home pregnancy test and it came back negative.. Have any other ladies on the pill ever experienced something like this? Should I take another test after the date i would normally get my period?