Period on birth control

I just want to know is it normal to have a period 10 days later after the first period started I’ve been on birth control for about 6 months, i have been off and on of it sometimes because I may forget to take but I had a period August 2-6 and now it’s the 16th and I’m having another one which is very weird to me and I hadn’t had sex for about a month so I know for a fact I wasn’t pregnant so I’m just really confused have a obgyn appointment coming up on the 28th. Just need some advice