What Causes Delayed Periods After Stopping 'yamin'?

i am 21 year old and i have problem in my menstual cycle...i took my doctros advice and she suggested me to take YAMIN..i took it for 6 months . during that time my periods was normal but when i stop taking it the next month it comes in time but the 2nd month again there is delay...what to do now.?? can why such delay is happening?
due to these delay i got lots of pimples..please help