yes, you can overtrain with cardio. the bad thing about it, you aren't lifting heavy weights, you are using much less effort, just a lot more often. So it can be hard to pinpoint what causes an injury. To me, overtraining results in loss of appetite and lethargy. If you don't feel better at the end of workout than at the begginning, you are doing too much of that activity. try doing a different kind of cardio every day and see if that helps.