it could be the food. is it pellets or is it mix? chinchillas' digestive system is very sensitive in that the bacteria in the gut takes about a month to adjust to new foods. this means that if you introduce new food, you do it gradually, slowly adding new food and reducing old, during a month period.
however, what is sometimes done with the rescues or with chinchillas you have no history of, you only give them hay (no pellets) for 2 days and then start adding pellets. you could try that.
if it is a mix rather than pellets, i'd advise against using that.
diarrhoea is not great. is the poo just soft, or does it stick together and onto shelves/fur? is it more like a thick puddle than pieces stuck together? you can get a product called fibreplex (for rabbits) - it comes in a dosing tube and the dosage for a chinchilla is about 1ml (that's the least you can dispense) - the dosage was for my chin who was about 450g then, your chin may weigh less. you can get this from the vet. however, if it is squishy poo rather than thick puddle, i'd try hay only approach first. if it's a puddle, i'd get fibreplex.
also, a chin takes about 12-15 hours to digest food so if you start giving hay only now, you won't expect any change for the next day (ish)