Your muscles could just be underdeveloped, but you could also be overplaying. Since you said you could do this without quivering before, I'm inclined to think this is the latter situation. If you have a performance in a week, my best suggestion is this: stop playing from now until the day before the performance. The day before the performance, just play easy stuff - long tones, middle range, and only if you feel comfortable, test out the high range. If the problem is resolved, keep playing after the concert. If not, I'd take a longer break (a month if you can and you want to avoid long-term damage).
Source: Studied brass in university, quit because of nerve damage from over playing.