My DS now 3.5yrs was very much the same up until recently. He still has a very high pain threshold and if he does cry I know that it's somewhat serious. But generally he gets straight back up and continues on.

I haven't experienced the stare but this may be her way of processing the pain, rather than crying.

The possibility that something may have been wrong with DS crossed my mind a number of times but I have always put it down to the extensive amount of invasive medical intervention he needed to get him where he is today.

Maybe make an appointment with your GP and if they feel it is needed, or on your request, they can refer you to a Paed (if you don't already have one).