Is it due to worms? Or there's sum kind of infection?

Regards....

Birds

If the tail is down and they seem to be straining to poop then could it be that they are egg bound and need help now from a avian vet but in the short term for immediate help place the birdies back end in a warm (not hot) saucer of water and then leave her in a warm quite place to drop the egg - if this is not the problem it could be that if they are chronic egg layers perhaps prolapse is a problem and you will need a avian Vet whatever happens.  Worming would also need a Avian Vet.