I didn't watch the show last night, but I did watch Katherine's salsa when I heard that she was injured, just to see what happened. She looked really uncomfortable throughout the entire dance - tentative steps, and the few times she smiled seemed to be when she remembered she was supposed to look like she was having fun.
There was no way that dance was a 9 - especially when you consider they probably marked her down for not ending it correctly.
This is IMHO: For her to keep apologizing for (a) being injured, and/or (b) letting Mark down is an indication to me that she has some odd issues going on inside herself. Any well-adjusted adult should realize that accidents/injuries happen, and the main thing is in recovering physically, not worrying about whether someone else will be upset because the injured person didn't do their best.