What Causes Absent Periods After Discontinuing Birth Control?

hi, i just want to ask about my menstruation. Im 20 years old,5'5" and 178 lbs. I have irregular period since 15years old. Last year, i took 2 cycles of birth control pills because i already have irregular menstruation. Its effective, but when i stopped it, i dont get any menstruation after that. Its been 10 months that i dont have menstruation. Im so worried. I also think that, maybe the reason why i rapidly gain weight is because theres a problem in my reproductive system. Please help me.