Suggest Treatment For Irregular Menstruation

I've been taking Lisinopril for 3 months. This is the first month that I've had an irregular menstrual cycle. It was a week late and now I've been spotting for a week. I normally have regular periods. I have taken over the counter pregnancy tests and I've even taken a blood test, and I'm not pregnant. Some days it appears I'm about to start 'flowing' as normal, but then I just start lightly spotting again. Could this be caused from my medication? I just had a pap smear in mid January and everything was fine.