Thank you for your question. Periods can be delayed for several reasons and strenuous physical activity can be responsible too. I'm assuming you are not sexually active otherwise you do need to do a pregnancy test to rule it out. I would suggest you don't worry too much.Keep a menstrual diary and observe it for a couple more months. It will most likely return to how it used to be. However if the problem persists beyond that do see your gynaecologist for some tests. best wishes