Why Do IStill Get Massive Breakouts on my Last Month on Roaccutane?

I've been on Roaccutane for 4 and a half months now. My dosages is 2 20 mg per day. My face and back cleared up within 2 months almost perfectly. Now as my treatment is about to end, I break out daily. I keep getting about 1-2 pimples per day but only on my face. Worst breakout was about 2 weeks ago during my menstruation. My chin area had so many pimples on it that it looked like it was burnt. I'm really worried that the medication is, for some reason, stop working for me. I'm very scared.