i gave my pup a handful of prawns in her dinnertime feed and tonight she's had the runs. Would the prawns have done this or could it be something else?

some one should be along soon with better advice , I can only say I would not give my dogs prawns ..hope pup is feeling better soon

Our dogs have prawns and are fine
but adding ANY new food should be done over a few days
otherwise it does upset their stomach.
Sounds like a case of a little too much too soon :-)

could be anything, mine eat the shells, (I eat the prawns) :D