I took plan b in february an since then my period has been irregular i sometimes don't get it or i get it twice a month. why?

Not the plan B: Plan b does not have a long term effect and it will not cause irregular periods all the way to july. There could be different reasons for your irregular periods such as hormonal imbalance ( most common one at your age group) , anatomical reasons such as fibroids or others . Please see your obgyn for an evaluation , Plan B which is a form if Progesterone is not the cause.