I've bad pain in tooth that had a filling, will i need a root canal?

Very likely: Pain after filling could be cause by many factors such as high occlusion (bite is high), post-op sensitivity, open margin (cavity is not sealed). However, if the pain is severe and right after the filling, very likely the nerve of the tooth is irritated to a certain level or preexisting nerve disease is activated by the filling procedure. Root canal is commonly recommended in this situation.

Possibly: Depending on how deep the filling was , it's possible to have a short term discomfort with cold etc. If you are having severe pain , or pain to heat them the nerve , may be involved. If its sore when biting , you may need to have the bite adjusted. Either way let your dentist check it out.

Maybe: First you need to see your dentist to determine if filling is too high and needs to be adjusted or if there is a problem with the nerve , if so, you will need a root canal.