On birth control, not having periods?

Before I went on birth control, my periods were highly irregular (I could wait 2 weeks for one, I usually waited for 4 months), hence me going on them. I've been on Yasmin for 3 years now without any problems, I take them at the same time every day, 3 weeks on, one week off, and I'm not ually active. Despite this, for the past couple months my periods haven't come. It has this time (and with a vengeance, ouch >_<) but is it normal for them to skip like this?