Can someone tell me what could have caused this feeling of fainting during my badminton training?

Even if the running wasn't tough it's never good to run then stand still. My track and cross country coaches always make us walk for awhile after our workouts to prevent problems like you had. I don't really know the technical problems for why it happens though. Maybe it's because your heart was still pumping really hard and your body wasn't moving enough to take in all the extra oxygen. Idk really haha