What Causes A Missed Period?

I had my period from January to June. Skipped in July. Had one in August and I haven't had one since. I'm a gymnast and practice 3 and half hours a week. I take 3 Ibuprofen 4 times a week for back pains. i can't figure out why i haven't had a period since August. This is the 4 year of having my period. Can you come to any reason as to why I've missed my period?