The lemon balm plant was receiving too much sun and heat. Also, it didn't help that it was a newly planted plant, probably in slight shock still. So I did a bit of research, and decided to cut the plant back to just a few inches high, and see what happens. The plant quickly recovered and grew new foliage, and its doing really well now. I think it was just a combination of shock, too much sun and heat, and the stress of it all.