First, I found a helpful website on tailbone pain:
The tailbone is often fused even in people without AS, but it does have several joints that have attachments, so it is possible to get enthesitis there. Also, a lot of pain here ("coccydynia") is caused or aggravated by sitting (or perhaps your monther-in-law as the case may be), so try not to aggravate it. Remember, take care of your tailbone and your tailbone will take care of you. Mine prefers Tempurpedic (memory foam) for both sleeping and sitting.

Second, Jen, my tailbone doesn't usually stand out as tender-to-touch type pain; that is only an occasional thing for me, but it does flare up at times. More usually I have major pain in the SI region (namely, the whole hip are above the tailbone including the lower back) and thoracic area (between the shoulders and lower neck). The SI pain does radiate to the end of my spine. In the midst of all that pain, my peripheral joints hurt too and sometimes one area may start to dominate. Falling asleep is very difficult without adequate pain medicine and more often than not a sleeping pill to make me drowsy. With that I am generally able to sleep until I awake to pain in the morning (First conscious thought of every day: "I've been run over by a truck!! ... Oh, no, I have AS..."). All that said, I think my tailbone is fused and very stiff, so it is easier to get injured (especially if you fall on it). Then I enter a whole new world of pain -- I can neither sit or stand for very long (future therapy might include launching me to outerspace to experience weightlessness, which come to think of it, "might also solve 2 other problems," according to my wife).

Jen, as for discerning the referred pain, it is difficult. But it is clear that I have numerous primary pain points -- such as the whole SI region and the achilles tendons -- where my disease is vicious and causing most of my problems. But the secondary pain can be felt radiating into nearby areas (even soft tissues or bone areas away from the primary joints). For example, the attachments to my heels are one of the real sights of attack where pain originates, but I can also feel pain up the back of my legs in the calve muscles. (Because this is soft tissue, for a while they even thought I had Lupus). I now understand that any pain I feel in the in-between areas is probably just radiating from the sorest joint in the region. Pain in my limbs is all on a spectrum (e.g., the pain in my elbows and wrists is worse than the pain I feel in between). It is not so easy to figure out though, because it depends on the proximity of joint structures and how complex they are; obviously the spine is under attack and where the shoulders connect for example just becomes one big sore region. The one that is most troubling is the mid-back pain that radiates to the chest and feels like a heart attack.

Well, another dissertation on pain.
DXed with AS 30 years ago