Don't Get Periods For A Few Months At A Time

I'm 18. When I was 15, I had sex for the first time, and not too long ago after that, I started missing my period. I just assumed they were associated with each other. The first time I missed my period, I had missed it for 4 months. I thought I was pregnant, but I wasn't. There were a few other times that I missed my period for a few months at a time but didn't worry about it. The longest I've gone without my period is 6 months. The last time I had my period was the beginning of November, so I'm about 2 months late right now. I hear birth control helps, but I can't buy any or see a doctor because if I use my card for that stuff, my parents can look on my account and start to question (because they dont even know I've had sex). I'm trying to make it to where they're the last people to EVER find out about any of this, so I'm just looking for some explanation or advice from anyone but them. Please help.