Achilles Tendonitis?

Treadmills after outdoors are notorious for Achilles pulls.
If the Achilles is tight, ice and a roller.
Do Achilles drops, and spend time on the calf muscles.
Strengthen the glutes, hammies, and calfs. This will save you.

But be very careful with Achilles soreness. Ice and rollers and strengthen.

Limit running in the interim while you figure out how bad this is.
Sheath pulls can take a while to heal.

Also, is the pain above the ankle bown, or below ?

Pain is about an inch above the ankle. Sensitive to the touch, and definitely worse in the morning when I wake up and take those first few steps, I assume because it's tight.

Progression was as follows:
-Trail run on Monday, felt fine afterwards.
-Discomfort started Tuesday night, continued on Wednesday.
-Thursday pain was much worse, and I spent most of the day limping, though it went away in the evening after I was forced to walk half a mile from my car to a restaurant for dinner.
-No discomfort Friday or all weekend.
-Tuesday the pain is back, though not as bad as Thursday.

Haven't done any running since this started, though Sunday was a short 5 mile ride. Have been applying ice. Given the lack of symptoms over the weekend, I'd hoped it would resolve with rest, but apparently not. Going to try to get in to see my old physical therapist tomorrow.