Temporary filling fell out after root canal and is causing pain. why is there pain?

Possible fracture: That fact that a temp filling fell out may suggest that there is a fracture in the tooth. A devitalized tooth is more brittle than a vital tooth and more prone to fracture. Of course, the temp filling may be unrelated to the pain. You've just undergone a surgical procedure and there will always be a healing phase for 3-5 days post-operatively. Consult your dentist pronto!

Temp Filling: If you just had the rct done, you may have some post-op pain for a few days. If it has been some time since you had the rct done, the temp failing can lead to a reinfection from leakage down thru the canals. Also, a fracture of root is possible which can produce pain and infection. It is best to go back to the doctor who performed the procedure and have it examined.

May be normal, but..: Most likely the pain is from the inflammation around the root of the tooth, especially at the apex or tip of the tooth where your dentist instrumented the canal(s). This is not uncommon and may last for a couple of days or more in some cases. The temporary filling has nothing to do with it. However, there are other possibilities, so call your dentist and ask for advice & a new temporary filling.

See your dentist: This may be part of the normal healing process. Have the dentist take a look at the tooth.

See your dentist: Regardless of the pain, you should see your dentist asap to replace the temporary filling. It's important to keep the inside of the tooth clean and free of bacteria.