Are there other teeth around this one or is it the only back tooth in
the area? Does it hurt when you bite down on it? If you run down the
stairs fast, does it make that tooth hurt? You can't go far wrong by
taking some antiinflammatory meds like ibuprofen...

Either the problem is your sinus or the tooth or both. It is normal that
the roots extend into the sinus cavity. There's often not alot of bone
in the area, and the second molar has long roots.
HTH
SP

Some dentists are afraid to extract any tooth.  Maybe yours is difficult.
Can't tell over the internet.  You *probably* need that or a root canal.
(but don't tell J**...Fruit Loops are supposed to be a breakfast cereal).

Fawks