Had tooth sensitivity sometimes especially after eating, had a filling today and 8 hours later still have the same sensitivity why?

Different tooth?: The tooth filled may not be the one that is sensitive. You may have something going on the same general area. New fillings can still be sensitive. That can last a few weeks to as much as a few months. That would still be in the range of normal.

2 problems: First problem is the need for a filling to be placed or an old one replaced. Both situations can lead to sensitivity especially if the filling is large or the filling is the first for that tooth. I would go back to the dentist and have them check the area. 2nd problem may be a little gum recession especially if the gum area is sensitive to sweets or cold for less than 10 seconds. See your dentist.

Sensitive tooth: After having a tooth filled there is usually a period of sensitivity to hot, cold, and direct bite pressure. The trauma of filling a tooth creates slight inflammation of the living tissue (the pulp) in the center of the tooth. This should be getting better day by day, not worse. If it is not improving it is possible there may be a fracture. Call the dentist if no improvement. Good luck.

Reversible pulpitis: Sometimes the nerve inside our teeth gets irritated and wants to swell. Since it is completely surrounded by hard tissue (the tooth) this soft tissue (the nerve) can not swell. This will cause pain on it's own. Does the pain go away with Motrin (ibuprofen)? This swelling is very slow to respond. The constant irritation from clenching or grinding my be the original cause of pain anyway.