Major issues. It is the area above your arse crack where that bone is. Very painful, at the moment I can't put on my own shoes.
I have a large torso and short arms. Extremely inflexible.
This sounds exactly like what I had. I had discomfort/pain in my extreme lower back above asscrack but to the sides. Got to the point my squats went from sets on 160kg down to sets below 100kg then down to nothing, then leg press started to suffer, soon after that I couldn't do anything comfortably and was reduced to the leg extension machine.
My problem was exactly like you, my body was so tight/stiff/sore with shit flexibility in legs, back, hips, glutes, groin etc, my hip flexors were so tight it had caused my entire hips/pelvis to tight forward, causing back pain, basically a combination of heavy leg work, no stretching and then sitting in the work van for up to 4-5hrs a day my body locked up and I couldn't function.
I was first treated for this in January this year, and it has taken 8 whole months to correct it and get squatting again comfortably. Treatment was religious stretching 3-4 times per day, massages, spas, epsom baths and no squats,deads, rows, leg press... nothing that loaded up my spine.
At first I wasn't satisfied, I was told "Lay off those exercises and STRETCH" so I did, and now, 8 months later it feels fantastic, everything has loosened heaps, flexibility has greatly improved, back feels great, hips are sitting square like they are supposed to and I'm back happily squatting again at last working my way back up.
Your story sounds awfully familiar to me, forget a PT at this point, go to a good Osteo/Physio and follow what they say to the letter. Get your quality of life back mate.