The Red Cross recommends that you refrain from strenuous physical activity for about 5 hours after donating. Examples of strenuous physical activity (> 6 METs) would include jogging, heavy calisthenics, and weight-lifting.
However, playing the trumpet would fit under mild to moderate physical activity. So you're fine. Examples of mild physical activity (< 3 METs) include working at your desk or a gentle walk. Examples of moderate physical activity (3-6 METs) includes a brisk walk or bicycle ride.
(FYI, a "MET" is a measure of physical activity.)
And as a couple people pointed out, be aware of yourself after donating. If something doesn't feel right, don't push it.