Hey there,
There's a good a chance you could be pregnant, giving that you and your boyfriend are having sex unprotected while he is ejaculating in you and the symptoms you described. Best thing to do from this point is to make an appointment with a doctor so you can get another pregnancy test done as well as a blood test (which will be able to give you solid results)
As for the brownish blood, it is old blood (as it is dark in colour). It could be implantation bleeding which is when the fertilized egg attaches itself to the uterus. Or it could mean that your period is due to appear, if the darkish bleeding is closer to your period date mind you.
My best guess is for you to contact your doctor or some kind of planned parenthood centre to get all the necessary tests done so you can figure this out.
Hope I have helped the best I can!