Collisons are very dangerous stuff. Make sure you ski in control at all times - meaning able to negotiate safely given the traffic. The person from the back who tries to pass bears all responsibility - that is the etique or code of responsibility on ski slopes. So people from the back must try all he can to avoid a collision. Sounds like you really understand that now. More importantly you have to learn to adjust your speed and do not follow too closely - just like cars on highway. I hope your problem will go away soon - sounds like some bruises on muscles from hitting the snow. Consider yourself luckly and speedly recovery!