Did you ever try to play music in a place that wasn't familiar, and found yourself so distracted by the room, or the people, or the lighting, or the phase of the moon, that you had trouble playing? Or did you learn to play music while sitting down all the time, and then try to play while standing up? It might have been uncomfortable at first. You were in an unfamiliar situation.
I was reminded of this two nights ago, when I flew my very first night solo (well, my first since 1971). I'd flown several times at night recently with my instructor, but hadn't tried it alone. So I took off about sunset and just practiced landings over and over, and kept at it as it got really dark.
Now, I've made about 400 daytime landings or so in the last 7 months. So I'm pretty familiar with them now. But now I was flying at night, and the situation was different. I really had to concentrate to find some of the same visual clues I'm used to in the daytime, and I had to adopt some new ones. But it worked. The results? 11 pretty good landings, including the last 3 in the pitch dark. But it did take concentration and practice in the new, dark situation (making those landings over and over). It was a gradual thing, but finally I was pretty comfortable with it. I really had to concentrate, but it just took some practice.
You can make the same kind of adjustment when you're playing music in an unfamiliar situation. If you're put off your stride (or even freaked out) by standing up to play, or playing in a new place, or playing in front of people, or playing in a group you're not used to, then don't concentrate on the unfamiliar stuff. Simplify what you're doing and concentrate on yourself and the notes you're playing. Keep your eyes on your instrument and play tunes you can play in your sleep, or your favorite basic backup licks, or just vamp until you have your hands and mind under control again. Let your brain assimilate the new variables a little at a time, and eventually you'll get used to the new situation. Play your same familiar tunes and licks over and over standing up, for example, and you'll get to where you can stand up in a group and handle not only your oldest material but new things as well. Practice at different places in your house, or your yard. Play when one or two family or friends are around-- not suddenly for a crowd, but gradually. Even if you're freaked out at first, it just takes some practice!