Well drinking 4L a day may indicate that you diabetes...id get it checked out!
As for the more frequent shits, have you been stressed lately? Coz when you got the fight or flight thing going on, your stomach goes into spasms and causes diarrhoea which i think is werid coz having the urge to shit aint exactly going to help you flight....or fight...well flight to the nearest loo lol.
But maybe youve developed a sudden allergy to milk or something? Or it could just be a gastro bug..you dont always have to get the nausea and shit with it, believe me ive had gastro so much aparrantly.
Have you been getting cramps, tiredness etc?
Maybe ring up the nurse health hotline..Dont know what youd call it there, but in qld we got the 13 health number.