Tremor is a common adverse reaction when taking Wellbutrin. It might go away as your body adjusts to the higher dose. But you should contact your doctor to be sure. Your doctor may want you to go back to 300mg.
Same thing happened to me. Told my doctor. He said it's a side effect from drinking caffeine while taking Wellbutrin. I switched to decaf, and the tremors went away almost immediately. If I overdo on chocolate (has caffeine), they'll shake a little. As long as you eliminate caffeine, the tremors will stop.