So how can my beer go from barely no carbonation from 3 days ago to practically shooting out of the bottle after cracking one open?
Depending on the position in the bottle carb timeline, this could be normal.
Did you chill both bottles for a day or two each before opening them?
It's possible your bottle carb process is working as it should and the second bottle you opened is just getting ready to start working the CO2 back in to the beer to make the real magic happen.
Wait another week or two before opening your next beer and see if your results are the same.