For 2 days, my lower canine tooth has severe constant/dull pain and sensitivity to pressure/tapping. xrays show nothing wrong. what should i do?

Pain in tooth: Is the pain triggered or intensify when cold or hot is applied? If the tooth is relieved from hitting when you bite together and the constant pain continues it may be too early for a problem with the nerve to show up on the xray. I would consult an endodontist.

May be nerve: Have your dentist test the tooth for vitality (is the nerve alive or dead)? Either way you might need root canal therapy, even if the radiograph doesn't show anything. Do you grind your teeth btw? This can also cause your problem.

Bite or RCT?: It could be either that the tooth is being constantly sensitized from clenching or grinding & the nerve is inflamed, or it could be that the nerve is irreversibly damaged & you may require root canal therapy. This often happens & does not show as a problem on the x-ray initially. If a bite adjustment does not help, you will probably need rct. Often it’s a judgment call without x-ray confirmation.

Trauma or fracture: If to cavity, no gum disease, and tooth sensitive to percussion, may be traumatic occlusion or fracture which may not have vislualization on an x-ray. If teeth out of alignment and canine in hyperfunction it can demonstrate the symptoms you describe. If toot is fractured, may not be seen on x-ray. Ask your dentist to check your occlusion and possibly refer you to an endodontist.