I Have Been On Birth Control For Quite Sometime. It

I have been on birth control for quite sometime. It completely stopped my period. I haven't had a period in about 9 months or so. My boyfriend and I had an accident the other day and I immediately took the plan b one step pill. I've never missed a pill of my regular low estrin birth control. I was antibiotics for 2 or 3 days when the accident occurred for a UTI. What are the chances that I'll become pregnant??