I just had vaginal surgery a week ago n now I'm 3 days late after having sex during my ovulation period... tonight I started but its only brown.. could I b pregnant

It's common to be late.
It's common to have brown (exposed to air) blood.
We can't possibly know if you are pregnant.
You don't even say what the surgery was, but it won't matter - no one can tell you. If I had to guess, I would say you are not.