Why is my period so off from where I chart it?

I other used to own 21 days contained by between cycles. Now (for times gone by 4-5 months), it's other 2-3 days stale from whenver I've speckled it to come. Why? (PS, I run track and cross country.)
Answers:

Should I tolerate my Boy Friend Ejaculate surrounded by my Buttocks? On top of it?

Your cycle can renovate bit by bit over time. or if you are floppy out next to a bunch of other women/girls your period will start synching up, exercise and diet also can effect your cycle.

I've gone from 30-36 time cycles to presently 28 days on the money.