is it normal?

So yesterday i started what i guess it's considered my period. It been brown ever since & it's been enough bleeding to fill up a pad so far. Is there any way that it can be implantation bleeding? I did have sex but it was always protected with condom & no breakage happened, we checked every condom afterwards. But since i had sex in my fertile week (protected) i was stressing alot on maybe getting pregnant. My period started 2-3 days earlier. & in my last cycle i was on vaginal antibiotics.