It could be a few things. First, you might have added too much flour. Always use the spoon and level method to measure (instead of scooping the flour out of the package with the cup), so you don't end up with too much. It could also be that you overmixed the batter. When you add the dry ingredients, do it in two batches and mix just until combined. Overmixing will develop more gluten in the flour which can make your cake dense. It's also a good idea to use room-temperature ingredients. Adding cold eggs and sour cream makes it harder to fully incorporate the batter, so you end up overmixing the batter.