Well the best thing to do is to drink lots of water. But if you start to vomit and there is no output whats so ever into the bag then that would be the time to go to hospital. I must admit that I did vomit just the once when I had my blockage but because I was still getting a small amount of output I told my self I would only go to hospital if I was to vomit once more or if the output stopped all together. Mine shifted by its self in the end though thankfully.
I have had my stoma for nearly 3 years now and that is the only time I have had a problem. Just remember that golden rule. "Chew chew chew"
I have never been hit directly in the stoma area(hope i havnt spoken too soon) but I would imagine that it would hurt. On saying that, I suppose there is more of a risk depending on what activities you do. I have read of a guy on another message board and he does all sorts of stuff from Rock climbing to Judo(is that the right word?) and he hasn't had a problem yet. I would say he is the most active person I know of. I don't even know anyone without a stoma that is as active as he is.