Why do sudden bursts of anger arise in deep Metta calm?

At some time, in deep, quiet Metta, I experience a sudden burst of unaccountable anger, which is strange to me. I must make fists to hold it in. Is it extreme anger, or is it an indication that the practice is causing the bypass to deeply hold resentment or is it an indication that the practice is causing emotional instability and causing extreme mood swings? My name is Aditi, DOB 30th September 1994. Time: 6:50 PM. Location: Delhi.