#18 tooth

I had the shallow mercury filling replaced with composite, 3 surfaces.
The only area that hurt right before the procedure was the rear-cheek
side crown area, when he tested with bite stick. I had been having
some minor pain (2/10) with chewing. Xrays showed that the root area
of that side was fuzzy; you could not see the "line" extending all the
way to the bottom. He felt the root may be problematic, but said he
wanted to try the composite replacement and to see if a crack was
there. Dentist took most of that crown off, said he found no crack.  

9 days after this procedure, I get minor pain (2/10) when I clench
down in that same area. It does feel abnormal when clenching. When I
floss between #18 and the rear molar, the pain is about a 5/10. Very
slight cold sensitivity, none to hot. Pain with chewing seems less
than before filling replacement. Now less than a 1.

Dentist called me and when I told him the update, told me to see an
endodontist for root canal evaluation. I have the appt this Monday.

Any thoughts?