If you are ill, don't even think about SW, focus on getting better. If you gain you gain, don't worry about it, you can always get it back off when you have recovered.
In that case, be careful of what you eat, but don't strictly stick to SW if you can't. If you can't move much cos of your back, you are stuck in a vicious circle really. All I can suggest is being careful of what you eat and hope for the best.
Could you ask your gp to prescribe some anti nausea tablets to take alongside your painkillers ?
My oh got some of these a few years ago they might help.