At what time of month is there the greatest risk of getting pregnant? Is it when I'm on, or at some other time? I've had a period 3-4 times during the last six months.

The biggest chance/risk of getting pregnant is when you ovulate, approximately 10 days before your period starts. But when you’re young and your period is irregular, you can ovulate at pretty much any time, which means that you can get pregnant at pretty much any time, including when you’re having your period.