Why does the cat meow when its going to the bath room?

Through my experiences with my 2 cats for meowing in the cat tray are:

1. They need to poop, which for some reason makes them have a mad half hour: they chase and attacks all sorts, even jump up the walls.
2. If they have had antibiotics from the vet and it passed through as they have a number 2, it is very painful for them.
3. Possibly they want attention.

However, if a cat cries when it is in the litter tray, it could mean that the cat has a urinary tract infection, or is constipated. Both these things will cause much pain for the cat, and will often cry out in a pained and distressed manner. It is highly advisable to take the cat to the vet, and in either case to increase the amount of water the cat is drinking, either by feeding more wet food, adding water to wet food, or giving some cat milk daily or putting some chicken broth in the cat's water.