Where's AF.?

Im 15 and I've been having periods for 6 months now. I usually start around the 20-22 ish (I'm still irregular I guess). Well here it is the 29th and no Aunt Flo. I'm not on bc and I'm a virgin so I know I'm not preg. I have a hormone problem ( over active thyroid) but I'm on meds for it and that's what made me start having periods. Is it normal to skip a month.?