Cold test on tooth, recently filled, hurt a lot, but nothing else indicates a problem. do i need a root canal, or could it be something else?

Healing: Especially if cavity was deep, may take considerable time to become comfortable. Cold sensitivity better than hot. Discuss this w your Dentist who knows you better than we do. Root canal treatment is an option, but may not be necessary. Give it time.

Cold sensitivity: Teeth can be sensitive to cold after new fillings are placed and this will ususally get. Better over a few weeks. And a root canal is not needed. If your sensitivity increases, or changes this would be more serious and may indicated a root canal

Get checked: Likely 3 things other than need for root canal. Often the bite is "off" or "high". This means that the new filling hits too hard and traumatizes the tooth every time you close on it. This can be adjusted. Also, when a tooth is restored, it is traumatized and recovery time is needed. The filling may also be leaking or not completely sealed externally or internally. Go back for this to be checked.

Return visit: If this was a deep filling, it may take some time for the tooth to accommodate the treatment. If it persists, a return visit to reevaluate would be suggested

No worry: Fortunately, tooth sensitivity to cold after a filling should improve on its own after a few weeks.