or should I be pissed?
Be pissed. Add me to the list of those who think you should "unfriend" her. She's using it as a way to gain access to your personal information. Cut her off!
I've had a similar problem with my psycho SIL who has taken to stalking me online. She has tried to follow be on twitter, and send me three friend requests on Facebook. (After I hit "ignore" for the third time, I got a message from Facebook asking me if it was an unwanted request. I said yes, and they've now blocked her, so good for FB.) Still, I have to watch what I say online at all times, knowing that there are some things she can still access. She can still read my twitter feed, just can't follow me. I refuse to give in and go private, so I still say things publicly, but have to be careful to NEVER mention SIL or NM anywhere that they might see it. I HATE having to live like that, but don't have much choice. This is the problem with social networking.
If you need to vent on FB, and want to be able to speak freely there, I would block your M. If she asks why, tell her. She's meddling in your private affairs. That's not what Facebook is for. N's won't back down until you force them to. It's never easy, but you have to do what to have to do. You're going through chemo right now and don't need this extra stress. If you want, you can lie and tell her that you're now limiting your Facebook activity, or that you're closing the account. Or actually close it, and start a new one using an alias. Your friends will understand.