Your agent should have done a better job of explaining. She didn't send the addendum to the buyer without your signature, did she? I don't know what your purchase agreement says, but generally, if the buyer's offer is contingent on an appraisal, the buyer would have the right to cancel the agreement if the appraisal is insufficient. The seller is not obligated to lower the price, but the buyer may cancel. Usually, most buyers and sellers negotiate something satisfactory to both parties, but they are not required to.
Bring up your concerns with your Broker, the one your agent works for. Hopefully your Broker can explain how your particular purchase agreement works and advise you what you can do.
I am a real estate broker qualified to advise you on California real estate matters, not the law. If you desire legal advice, consult with an attorney.
Hope this helps!