The period being regular for all 3 months while you had unprotected sex, home pregnancy
test being repeatedly -ve- there is no chance for pregnancy.Still if you have doubt,blood HCG testing/pelvic sonography
should remove your doubt.
The brpwn discharge may be starting of your period,
Enlarged tummy can result from-various conditions including-gases/constipation
,fat deposition,intrauterine conditions like fibroid
and so on.
You need a physical examination by a doctor,and investigations according to need ,for pointing out the causative factor & treatment accordingly.