There could be a number of reasons why your period would change its usual cycle. Stress, recent weight being lost or gained, a change in eating habits, exercising, normal hormone changes, and even late ovulation.
I'm not sure if sex would cause it, but I did notice that after I lost my virginity, my cycle changed a week or two on me. I wouldn't give it a second thought- it sounds to me like you're fine and everything will balance itself out!
Happens to me too. If im not sexually active my periods are fine but if i do have sex for the next few months they are a bit off. Don't worry about it i am sure you will be fine.
This is something that I thougth I would keep to myself but I find that my period can "move around" due to anything sexual. When I masturbate my period may come on a few days later. Also if I have sex before my period comes on it comes a little later sometimes and if I have sex while it is on then it may stop a few days earlier. Doesn't bother me much (unless I needed it to come on the first date) But I would say its normal because it happens to some of my friends as well.
Thank you, ladies! That makes me feel much better! :)