Is it normal to have two periods in one monthand what can cause this. i normally only have one a month and i'm on birth control?

Once a year?: A woman with cycles shorter than 29 days (shorter than 1 month) will likely have 1 ovulation per month during the year, but have 2 ovulations during one month of the year (since she has about 13 cycles a year).

Sometimes, yes: As long as this is a transient, as opposed to recurrent, occurrence. This can especially become an issue with ocp (oral contraceptive pill) transitioning.