Other things to consider...
Had the fielder already received the ball ahead of the runner getting to the plate, then was she standing there stationary, waiting for the runner to get there to apply a tag? (Then the above noted "crash interference" rule might apply.)
Did the ball, runner and fielder all get there at about the same time. (Then the crash interference rules might not apply.)
Did the fielder receive the ball out of the base path, then suddenly move into the runner's path to make the tag? (Then the collision wouldn't be the fault of the runner and she shouldn't be penalized for it.)
Did the runner in any way slow down, attempt to go around the fielder or alter her path in any way to avoid the fielder in possession of the ball? (Then she may have met her obligation to "attempt to avoid contact".)
Did the runner slow down, attempt to go around or alter her path in any way due to the fielder's presence before she had possession of the ball? (Then you may have obstruction on this play.)
Did the fielder actually touch the runner with the ball, then lose possession after that point? (Then this would be a valid tag and the runner would be out anyway. So long as there was possession of the ball at the moment the tag was made, it doesn't matter if the ball was lost after the tag was made.)
Any one of these details- and usually a combination of more than one- could influence the call. Now you now why we say these kinds of calls are "had to be there"!