She may be choking - she may have something stuck in her throat. They can overeat. How is her breathing? If she gets to the point where she cannot breathe, gently hold her head and straighten her neck, tilted down slightly so she can get the blockage out. A flexible tube might be needed in case of emergency, if she cannot breathe. Storey's Guide explains the process.
How old is she? Has she had any grit?
Before giving peas or other treats, ducks need to have had some grit. If she is a duckling, then a teaspoon of chick sized grit in her food once a day for several days (need to check Storey's Guide for the details) before having treats like peas.