Do Periods Stop-Start?

So my girlfriend and I had anal sex(I didnt cum in her) on june...her periods were supposed to start by the end of july..but it didnt...she took progesterone tablets(Deviry 20) prescribed by her doctor in past...and she got her periods which were delayed by 15-16 days...she had her periods from 12-17 august , she mentioned tht the blood flow was comparatively lesser than normal..she also said tht the blood was deep red in colour and had clots...then after 7-8 days on 25th aug she is bleeding again and she said there clots present in the blood. Like can periods stop and restart again like this?..is this normal?...like we are worried about this being implantation bleeding so we r very confused and stressed.
Please someone lets me know about this..