Period is really late?

my period is almost 4 months late and its really bothering me. im 15 years old, had my period for about a year, and im a virgin. my periods still irregular, but the latest its been before is 2 months and now its almost 4. i mean, dont get me wrong, i dont mind not having my period, but its just kind of bugging me that its this late and i dont know why. is there something wrong? ive been using this topical acne treatment called tretinoin, could that have anything to do with it?