To me, it sounds like you just may be headed in the way of your periods stopping altogether. This would be due to the hormones in the pill. It's not as common with the pill as it is with the Mirena IUD or the Depo shot, but it's not unheard of. Also, being 18, you're still going through some body changes, and even without any birth control at all, sometimes there will be times of irregularity in your period. The other possibility is that you've been losing weight.