You are not updating the lastPos at the end of your mouseMoveEvent ? From the Qt example:
void GLWidget::mouseMoveEvent(QMouseEvent *event)
{
int dx = event->x() - lastPos.x();
int dy = event->y() - lastPos.y();
if (event->buttons() & Qt::LeftButton) {
setXRotation(xRot + 8 * dy);
setYRotation(yRot + 8 * dx);
} else if (event->buttons() & Qt::RightButton) {
setXRotation(xRot + 8 * dy);
setZRotation(zRot + 8 * dx);
}
lastPos = event->pos();
}
I suggest you take a look at Qt Hello GL which seems to match your use case.