This may clear up in which case it might not be anything to worry about. Review your sanitary arrangements and be extra vigilant and ensure there’s not some obvious cause such as mouldy feed. If posisible isolate the bird. If the condition continues beyond about 4 days or there’s a drop in condition and you suspect a digestive infection or food poisoning you may have to consult a vet for a more definitive answer. Digestive problems can be difficult to diagnose and some history about the birds breeding maybe useful to know in forming a diagnosis.
Normally I'd say don't worry however given the age of the bird and your description of the symptoms this would not be untypical of Lymphoid Leukosis. A disease which is generally vertically passed through the hatching egg from hen to chick. It is characterised by a long incubation period of 4 to 6 months and frequently occurs at times of stress such as when laying starts. Other signs are emancipation (compare the breast bone and flanks with your other healthy birds), loss of appetite, low production. Horizontal spread is usually low so if you isolate this bird and there's every chance even if it is LL your other birds will be OK. Keep under observation and particularly check for any signs of prolapse due to the enteritis, treat for dehydration if the diarrhoea persists.
You can make a very good electrolyte using the following formula. Make a solution of 2ml salt substitute (potassium chloride), 3ml baking soda (sodium bicarbonate), 5ml salt (sodium chloride), 10ml glucose (optional) in 1 litre of water given in place of drinking water for seven days. Chickens seem to like apple cider vinegar added to their water so you can add between 5 – 10ml per litre of drinking water to encourage them to drink. Adding a multi-vitamin supplement and using a higher protein feed such as ex batts pellets (rather than layers) can help with convalescence.
Regards