My Dog Has Dark Brown Discharge And I'm Not Too Sure If She Is Pregnant. I Haven't Had The Chance Of Getting Her To The Vet. She Did Mate With Another Dog. What Might Be Wrong?

This discharge does not sound normal.  Once a dog gets pregnant it does not start discharging a brown discharge.  Right before delivery a mother can pass a mucous plug but this should not be a chronic or ongoing discharge.  It would be best to have this dog examined by a veterinarian.  They can look at this discharge to see if there is an infection present.  Pyometra or uterine infection is a serious and potentially life threatening disease.  
When did she mate?  Pregnancy in dogs lasts 63-66 days.  So if you know the date of breeding this should help you decide if your dog may be pregnant.  Pregnancy can be diagnosed via palpation at day 22-26, by ultrasound examine day 20-22 and most accurately at day 26.  The fetal skeletons calcify at day 45 and can be seen on x-rays.  Please talk to your veterinarian if you have further questions.

It is absolutely normal for your dog to have some form of discharge but in case of the brown discharge, this could indicate infection. I would suggest you get her to the vet without delay as this also can help you rule out if she is pregnant. Good luck.