When I first started using a pick, I played downstrokes exclusively for several months. When I first started mixing in upstrokes...i noticed mainly that I didnt' really have enough strength in my hands to do it properly. So as a result I would tense up.

What I'm really trying to say: Just give it time, try to be relaxed but if you have to force yourself to relax extra, then just don;t worry about it for now.