What Causes Vaginal Spotting While Using Birth Control Pills?

I have been on birth control for 3 years now. I was on Aviane for 2.5 years then got switched to Orsythia by my pharmacy for about 6 months and have since gone back to Aviane. I just started the 3rd week of orange pills and yet I'm spotting. As I've never spotted while on birth control and it's too early for my period, I'm wondering if this is normal or a sign that I need to start a different birth control.