This is your business and you need to react pretty quick to nip this in the bud. This ladys attitude is unacceptable and merits disciplinary action. I believe that you need to speak to her, in private away from other staff members, informing her that you have noticed a change in her attitude, is there anything that is troubling her ? is there anything that you can do to help her ? Explain that you do not wish to lose her as an employee however you need to see an immediate change in her behaviour. Tell her that although the conversation is verbal you will record the date, time and content of the conversation and keep a copy of it in her employee records. Obviously if things do not improve then a written warning would very soon follow, then dismissal.
Best of luck,
Conrad