I agree with the last two posts - however, be careful with the suggestion that you should keep your elbows glued to your sides, because the idea is to release the bit (and in turn the horses mouth and head) when you jump. With your elbows at your side you are not going to be able to get into propper position to make the effort comfortable for the horse.
One of my students had a similar problem, but seemed to do it all the time (wasn't advanced enough for anything more than cavalletti)... I had her ride - a little uncomfortably I'm sure - with her wrists literally touching one another for the most part of a couple lessons... she seemed to relax a little more in the upper body when I told her she didn't have to any more and the problem corrected itself.
Good luck!!