A question about sitting, movement and mindfulness
First of all I think it is important to remember that in the Mahasi Sayadaw system of meditation, you don't necessarily have to note distractions. When distractions arise you have a choice between staying with the primary object (e.g. the movement of the abdomen) or you can take up the distraction itself as an object. Both of these are equally valid. If the rocking motion is coming up a lot, you might decide to ignore it and stay with the abdomen. If it only happens sometimes it might make more sense to note it, but it makes less sense if it is happening continuously.

Also, if it is painful for your body to sit on the floor or on a cushion, I would recommend that you try meditating while seated in a chair and see if that is less painful. As long as your position is relatively stable and able to be maintained, there is no problem, and sitting in a chair fulfills these principles, so there is nothing wrong with sitting in a chair to meditate.