Lamictal induces the metabolism of estrogen (which is why oral contraceptives may not be as effective, especially low dose estrogen ones) and that is probably why your cycles are irregular.
You should talk to your doctor about what to do, sometimes it might make sense to take an oral contraceptive to counteract this effect of Lamictal.
You should also definitely talk to your doctor about getting pregnant, this is something that takes some thought (balancing risks and benefits of various approaches to treatment during the period of trying to get pregnant).
Hi I have Bipolar but I have never taken Lamictal, but I am on Sodium Valproate and have been on Lithium aswell. I do know that Lamictal can cause menstrual irregularities, I'm not suprised to hear that your cycle has got longer and that it is lighter.
I know that with this group of medicines pregnancy is contraindicated and birth defects have been reported. I couldn't be exact about Lamictal but I would definitely stop taking it immediately if I was to become pregnant.
You need to discuss methods of birth control because Lamictal can react with certain contraceptive pills etc.
Goodluck, and keep well :)