Good answers so far, but there are many factors that can cause the problem you're having.

As others have said, it could be pilot induced (Giving too much elevator) or wind induced (Turning into he wind which can create more lift)

But a third possibility is the fact that as you turn to final - while losing altitude - can result in an increase in airspeed. The increase in speed may not be noticeable while turning because the extra lift that is being generated is helping the plane through the turn. But when you level the wings, the extra lift kicks in and creates an effect known as "Ballooning".

Ballooning is most often seen in trainers, or any aircraft whose wing has a high lift aspect.

To avoid it, try reducing the amount of decent in the final turn - in other words, don't give LESS up elevator during the turn, give MORE.

This will reduce the amount of altitude you lose, which will keep you from gaining too much airspeed.