Jaw popping after having cavities filled. Is this serious?

Disc. The disk in your jaw joint may be out of position from being open so long. Eat soft foods, don't chew gum, rest the joint, warm compresses 1st 24 hrs, cold compresses after that. Notify your dentist. Probably will resolve in a few days.
Temporomandibular. May be dislocated, see dentist, and get it popped in place.
Disk displaced. Your TMJ disk out of position. Mouth probably stretched wide during procedure. Serious or not not known for awhile. See dentist.