Lamictal dosage

I have Bipolar I, and have been experiencing a great deal of relief from Lamictal/Lamotrigine for several years. However, my psychiatrist is starting to raise the dose, and I don't know why. I haven't been with this psychiatrist for very long. I was on the usual dose of 200mg/day; then, she raised it to 400mg/day. I'm not sure I felt much better from raising it, but it's possible, so I wasn't worried.
Now, though, she wants to raise it to 600mg/day. This is far above the recommended dose. I am on a contraceptive, which I know sometimes decreases the effectiveness of Lamictal, but it's still an extraordinarily high amount. When I called to ask her about it, she was very defensive and wouldn't tell me anything, not even why she wanted to raise the dose. Now I am worried! What should I do? Have any of you ever heard of someone taking that much Lamictal per day?