i have a female guppy who has a large bump on the top of her body, she also has a small white string which has a little ball at the end hanging down from her anal, her tail also bends down.

does it have a disease that can be cured or should i put it out of it's misery.

Can you describe the ball please, does it look like mincemeat with white bits in it,
or does it look like its intestines.
The lump internal parasties can cause bacterial infections.
The lump does it look hard or soft like filled with fluid.
Has the lump just suddenly appeared.

sounds to me like youve got worms which may have died in her intestinal tracts and shes tried to pass it through her body.

i only say its dead because usually a worm wont live inside the body unless its got food, you could try meds or even fasting your guppy to kill it if it is worms.