Kitten changed diet,runny stool-vet techs/cat experts-what shoukld i do?

i bought a high quality kitten food,grain free,etc-but it didnt agree with my kitten.she is lively and drinking water,but her last stool(a few minutes ago)came out runny/gy.if it continues,i will take her to the vet 2morro morning.for now,is there anything to help her?i found her at 5 weeks old,abandoned.she always refused the bottle,so she has been on wet kitten food.dry food doesnt interest her,and seems hard for her teeth.she has seen the vet once.been de-wormed.has appointment for sept 15th to receive vaccinations.how worried should i be?for tonite,what can i do?