Suddenly, when attacked I am unable to fightback so I just keep taking damage and stumbling back. I can't use magic or weapons. I have no idea why this has suddenly happened. It first happened at the end of Following a Lead, and I had to let my Black Hand buddy kill the traitor while I took damage and replenished my dwindling health with potions.

If I can quickly get a magic attack in, I can kill weak creatures like rats easily, otherwise I'm doomed.

Is this just a glitch in my pre-owned game?

I think its a glitch.Try to restart the game or start again from last save.

Major Agility drain? What you describe looks like you're being constantly staggered and that normally means very low agility. Do you have a disease, or have just encountered a creature that might have cast a damage agility spell at you? Check the stats and magic effects tabs to see if that's happened.

As another person said look at your stats. If your agility is very low from a disease or damaged, especially if it hits zero, what you're describing is very possible. If it's in red visit a shrine. If you have too much infamy and it won't let you visit the church? Use restore agility potions. You can find them in numerous alchemical shops.