My Mom entered a nursing home for long term care. Does my health care proxy still count?

This is what I would call a "bait and switch", because they accepted her knowing what her medications were, and arbitrarily made changes. Definitely give their MD a piece of your mind and ask him if you need to file a complaint with the joint commission. (jointcommission.org)

Thanks to everyone for your answers. The Director of Nursing told me she's "just a nurse" and that the doctor makes all the decisions. So I will address it with the doctor. But still, there's something criminal about holding back the only available treatment for a patient's medical condition without fully informing the patient or their proxy. I'm going to talk to my lawyer about this.

By healthcare proxy you mean that you have the power to make medical decisions on her behalf if she is unable to do so on her own?

No, this doesn't mean that you can decide what medications she can and can't be on while in a nursing home.

If you don't agree with the nursing home putting your mom on or taking her off medications talk to the Director of Nursing.

If they admitted her, they must have seen her medical records, yes? Why would your health care proxy no longer be valid? Make an appointment with the director of nursing to find out who made these changes and why. Your job is to advocate for the best treatment for mom. Listen to what they have to say, but do your own research.

∆ Unnecessary Drugs.
Medicare and Medicaid regulations address the use of drugs or medications to treat clearly identified medical conditions in the nursing home. Possibly the most important concern is the overuse or administration of unnecessary drugs. The problem is addressed by regulation §483.25(l) Unnecessary Drugs as follows.

1. General: Each resident’s drug regimen must be free from unnecessary drugs. An unnecessary drug is any drug when used:
(i) In excessive dose including duplicate therapy; or
(ii) For excessive duration; or
(iii) Without adequate monitoring; or
(iv) Without adequate indications for its use; or
(v) In the presence of adverse consequences which indicate the dose should be reduced
or discontinued; or
(vi) Any combinations of the reasons above.

2. Antipsychotic Drugs: Based on a comprehensive assessment of a resident, the facility must ensure that:
(i) Residents who have not used antipsychotic drugs are not given these drugs unless antipsychotic drug therapy is necessary to treat a specific condition as diagnosed and documented in the clinical record; and
(ii) Residents who use antipsychotic drugs receive gradual dose reductions, and behavioral interventions, unless clinically contraindicated, in an effort to discontinue these drugs.