It's most likely just stress, some individual pigeons can be like this, usually when separated in the winter period. Check their crops 15 minutes after feeding and they will be full of water, the bird becomes slightly dehydrated after passing all this fluid and the cycle continues, as I previously advised giving clay pickstones will help as the salt levels in the body will be replenished and the clay content helps the detox process also helps the gut fight against coli/salmonella, also contains certain beneficial minerals, above all the birds love it. Carob powder as I also suggested is used to help bowel problems diarrhea etc, you might think this wont work because it is to simple a remedy, but don't be too quick to rush to the medicine cabinet just try it.