Is it possible to start a period at 12 dpo but still be pregnant. i o'd on 8/25 and my period has started today.

Not likely : Probably not, but i couldn't say for 100% sure... it could happen but It would be extremely rare. If you really want to be sure wait 5-7 days and take a pregnancy test, but like I said, it is unlikely.

YEs: Yes, it is posible to have vaginal bleeding after pregnancy has been established. An old adged I was taught was if a cycle is too little to early to late, that pregancy should be excluded.