Asking for my girlfriend. Period date?

Hi. We're having a confusion here. Any help will be appreciated. Last Intercourse we had was on 1st of Aug. We used condom and ejaculated outside, every time. Her last period started on 19th of July. Next expected date was 17th of Aug, but since generally she gets it 2-3 days later. Considering this, her periods should've started by 19th of Aug. But she experienced fatigue, cramps and other PMS's. And finally when we thought she missed her period, she started bleeding on 29th of Aug. We are confused, whether it's an implantation bleeding or periods. Her flow continued till yesterday. Today also, little of bleeding took place. You can't call it spotting, because pads filled up and a small clot was also found. Color of the menses was light red to red. Should we be worried that she is pregnant? Can implantation bleeding occur after 10-12 days of expected period date?