Hi, Lindsey:
A couple of thoughts: how long has your plant been in that pot? Lemon balm grows roots like crazy, so it's possible it's rootbound. In general, herbs don't need much fertlization, but if it's overgrown in it's pot, it might not have many nutrients left. The other things to consider: is it getting consistent watering? Is the glass from the window getting too warm and scorching the leaves?
You may wish to consider putting it outdoors in a sunny, but somewhat sheltered location for a while. It should do very well outdoors in your zone.
I grow my lemonbalm in my garden, and just routinely chop some out to keep it contained. You can also bury a pot outdoors to keep it contained. It is a perennial in our zone.
Linda
