Thats normal. The bottom leaves are shaded and don't get much light. Even OD when its all bushy. So the plant sends all the food and water to the top where the most light is getting too. Keep an eye on it though and make sure it doesn't progress to the middle upper growth of the plant. that would be a sign of a problem.
If you can snag a pic and post it do it. That will let us know for sure whats going on.
Also post you fertilizer type and schedule along with how often you water.