V_AC = 3.31 V
V_BC = 1.33 V
V_AB = 0 V
The key measurement is the voltage across the resistor. That will tell you if any current is flowing. It is possible you will find that value is also zero (although I suspect you won't).
You might then ask why 0 + 0 = 3.31, and my answer is that you may have a white led, or some other LED with a Vf greater than your supply voltage. Thus, effectively zero current flows and the loading by your multimeter measures the voltage drop across each component as zero.
Another option (and the one I'd lead towards) may be that you have killed your LED and it has now failed short circuit. if this is the case you will see the voltage across the resistor as the same as the voltage across A and C. It would also explain why the voltage across B and C is so much higher than zero when the output is presumably pulled low. For this to be the case, I would expect the value of R1 to be under 100 ohms -- possibly well under that value. What value is R1?
Do you have a diode check function on your multimeter? It should show the forward voltage drop (if it's low enough for your multimeter to measure) and light up dimly during this process. If it shows a near zero value, the LED is shorted.
Another option is that you have a wiring error on your PCB or a programming error on your MCU (for example, having it set as an output instead of an output, but I'll discount those for the moment.