I am being careful

To use a urinal while in a wheelchair, one must edge toward the front of the chair, so that everything is, as my doctor said, "descendant." At least I need to.

This means undoing my seatbelt.

Until recently, this worked fine.

Twice in the past week, though, I have coughed after I set the urinal down but before I redid the seatbelt.

The results have been more or less obvious. I fall onto my upper legs and then take a header onto the floor. Actually, I take that back. The first time, I somehow steered my chair toward a little table in my bathroom and rested my head on it until my sister rescued me.

Not sure why this is happening or what to do.