I Was Doing Conditioning For High School Football Today And

I was doing conditioning for high school football today and I really pushed myself, I felt exhausted like I should be. After that our coach let us break for 5 minutes then I took a knee for like 5 minutes while we were discussing formations. Once I stood up my vision completely blurred and I couldn't see a thing, I felt weak and struggled to walk 40 yards to the medical center, everything was fine after 15 minutes. What was this and what can I do to prevent something similar