Blueberry Leaves Dropping

Hi, I received a blueberry plant for Christmas, and it grew at least a foot over summer/autumn. One branch in particular grew much higher than the rest of the plant. Now these upper leaves are turning red, the veins followed by the leaf itself. Then they drop off. The lower leaves are still a dark, glossy green. I have fed it with a liquid fertiliser occasionally, the recommended amount on the fertiliser bottle. Any ideas? Thanks!