Got a deep filling 3 months ago in a molar & tooth was completely fine until one week ago, extreme sensitivity while having hot or cold drinks, no problem while chewing. i wear a mouth guard too at night for grinding, can that put pressure?

Did the dentist tell you that the cavity was deep and near the nerve, and if it acts up in the future you will need a root canal?. You have the situation of hot and cold sensitivity, that are the signs that you need a root canal if you want to save your tooth. The night guard is to prevent you from grinding your teeth and wearing away the enamel. Get a root canal or you will lose your tooth.

Have your dentist re-evaluate that tooth... large fillings can sometimes loosen or crack and need replacement.

Nerve of tooth may have been irreparably damaged by the deep cavity (infection). Return to your Dentist for evaluation. Root canal treatment and crown (cap) may be required.