Ask a Dentist. Get Answers to Your Dental Questions.
Your assessment of "hard" does pretty much rule out fistula/infection/abscess, etc.
This whit hard spot, behind the last molar, is "often" keratinized (hard,toughened up) tissue/gum. Generally in that area of the mouth it is often inadvertently caused from accidental biting from the upper opposing molar. Something that occurs in many of us, and happens quite frequently. It will likely not be an issue...but since you are about to visit the Dentist I'd ask him to look & ascertain his/her expertise to confirm what I've mentioned.
I think the only real issue of magnitude, or at least in priority is the decay under the filling, which likely is the pain culprit which now bothers you. So, you are being prudent to get it fixed today. And, while you are there, ask the Dentist about the white hardened "likely keratinized tissue" gum area. I hope this helps you!
*PS: Hope all goes well for you today. Let me knoe if you have additional concerns - OK?
Generally keratinized tissue is rather rough and toughened up, and has little to no pain, so generally not painful to the touch. Is the pain you are having coming from the white area of the gums or the tooth with the decay?
I may have misunderstood you. I was under the impression the rough/keratinized area was directly behind or to the rear of the back molar. Since it is more on the tongue side, true, it could be nothing of significance, but DO have the Dentist check it out when you are there today. Pus or fistulas occur fairly rarely on the tongue side. So does the keratinized tissue (occur rarely) on the tongue side of the tooth. So, I'm essentially "unsure" what it is, and it would behoove you to have it checked today, when getting the leaky filling replaced. I hope that makes sense.
Let me know how your Dental visit goes today, and what the tongue side area problem is esteemed to be - per your Dentist, if you care to. All the best to you today!