Reading your post reminds me of when I had a deeply filled back molar turn nasty and slowly die. It didn't show up on Xrays, and eventually I had an episode of locked jaw...which lasted 2 weeks.

I went to several specialists, and TMJ guy said, in his opinion it was not TMJ. (I had xrays of the jaw joint from him) Finally I had it fixed... root canal and crown. It took about 6mos to settle down. It is fine now.
The last tip off was a routine cleaning, which sent me almost out of the chair. But I had no abscess or other visual signs. Vibration and cold from driving the car finally became very painful too. I did get several sinus infections from this tooth, as they all stopped when it was fixed. (it was an upper back molar)

Mel, you are diabetic. If you have some smoldering infection brewing there, eventually it will affect your blood sugar which will go up.

If I were you, I'd get another opinion. It took 3 specialists to finally decide to get the root canal. Even then it was a toss up.
Any further swelling or worsening should be a sign for you to get more aggressive with this situation.

That is just my opinion based on my nightmare years with that tooth!