I guess the first thing to look at would be the number of wet nappies. If they seem dry I'd get off to the dr.

are you able to offer the breast between formula feeds through the day? Maybe she's thirsty and formula is a bit too heavy for her? A quick drink at the breast would quench the thirst without having to have a 'meal' as such.

Sorry, don't have any real advice, but I'd definitely keep an eye on those nappies.