My tooth cavity stopped aching , 3 days later there was a bump filled with blood and pus .the same thing happened on my 1st cavity . what is it ?

An abcess: Most likely the pain went away when either the nerve died from the pressure or the pressure of the infection broke through the gum. Either way you will probably need a root canal treatment or an extraction. Even though the pain is gone don't put off getting treated. You would still have at least a low grade infection circling through your body.

Tooth fistula: It may be a tooth fistula. A tooth fistula forms in response to an infection at the base of the tooth's root. They are channels that form within the bone and gums to give infection an avenue for drainage. Treating a fistula requires antibiotics and/or possibly extraction or root canal. To have a fistula correctly diagnosed and treated, consult with a dentist as soon as possible.