I have a gross odor and a strange brown-ish discharge coming from my vagina. What is this?

Possibly an infection, could be what remained from a previous period, could be a period about to start. If it continues, it would be wise to see your doctor for an exam.

Actually this is more likely to be BV - bacterial vaginosis - which is essentially an overgrowth of you OWN normal vaginal bacteria that has overgrown do to an unbalanced pH - and you might need an antibiotic such as Flagyl and use of a Probiotic.
I hope this helps, thank you