I paid off my Mom's mortgage. Now she is contesting it. Can she take the house back from me?

If you recorded the quit claim with the court house there isn't a lot they can do. The full name is a quit claim Deed And if recorded the property now belongs to you. Most counties now have this information electronically uploaded and you can search a copy to see if recorded on Netronline.
I suggest a financial advisor at this stage. Transferring property back and forth may flag you for an audit and there are state and federal transfer taxes associated with each transaction. It can get a bit hairy. Also, if kept in your name you will have to pay tax on money distributed to siblings . Proceed from the sale may put you in a higher tax bracket. All that aside I'm convicted that money will tear a family to shreds so if you have siblings get them involved fully now. Put all cards on the table for all of them. Good luck!! All will work out in the end.

Wow that is sad to hear ,but I am wondering is it possible someone else is pushing her to do this? Perhaps another sibling ?

What a horrible thing to do to you. Was the amount that you paid off a substantial portion of the cost of the house? Do you have a receipt for the payment. In your situation, I would consult my own attorney. You may want to sell the house back to her for the amount you paid, including the interest you lost when you took out the equity loan. Or you may want to contest the return of the house. Does she state the reseason that she is contesting it?