>>The two ostensibly break up after J.D. leaves her on a page in the middle of a date in "My Drama Queen."
This isn't quite true, as at the end of this episode you see J.D. and Jamie kissing and J.D's voice over says "but once you get past the drama, it's actually pretty simple." But she still seems to disappear after this (although I believe the break up is mentioned in Her Story II).