Nicki
You could try a trick I do when I'm preparing to sing - obviously you'd have to put the flute down first
Stand facing your music stand, with your feet slightly apart and your hands dangling down by your sides. Drop your shoulders down and back. Give a huge sigh. Feel all the stress and tension in your body seeping out through your feet into the floor. Stay like that for a couple of moments, feeling yourself relax. Close your eyes and make your face blank, so that you are totally relaxed. Just breathe in and out a couple of times.
Then, when you're relaxed, pick up your flute and try playing. If the tension creeps back, put it down again and do the relaxation again.
Hope this helps
Cheers
Katyjay