Is Light Spotting Periods Normal While Being On Cilest?

I am currently on the birth control pill, Cilest. My periods have been extremely light, spotting at best. I have taken the pills almost perfectly--never skipped a pill and have taken it two hours at the lastest. My husband also had a vasectomy five years ago, and his sperm count was zero. Is it possible I could be pregnant, or is this normal while on the pill?