The power of attorney is not legally binding on you. All you have to do is file a revocation and it is done. You should be able to just pick up your child at any time and take her home. You mother could be charged with kidnapping if she doesn't return your child. (Do both you and the child's father want her back?). If the police won't help, you might have to file a habeus proceeding with a Court, but it shouldn't go that far.
Good luck to you.
I don't practice in Texas but I would be pretty sure that there are legal remedies for a parent in your situation. Because you are faced with an emergency, you probably should not try to handle this on your own. You need an experienced child custody attorney immediately.
Don't wait for an answer about what you can do. Use the resources for finding an attorney in Houston that the Avvo staff posted. I'll link it again below.
Another resource would be looking for a parent-serving family law attorney at the National Association of Counsel for Children attorney referral site, which I will also link below.
In Massachusetts and any other state using the Uniform Probate Code's guardianship of children provisions or anything like them you are right that a notarized power of attorney is not a guardianship. Any authority that a parent gives to someone in a document that is not turned into a guardianship or other form of custody by a court is revocable by the parent.
If she doesn't give your daughter back based on your request be sure to get a lawyer and get into court before she leaves. You might also consider contacting the police for the town where she currently resides as her refusal to give you back your child or to take her out of state without your permission MAY be a crime.