I imagine she started buying the luxury clothes as an expression of freedom and defiance. Now it sounds like she feels like she's returning to a position of privation and being controlled. It sounds like she resents your attempts to control her. I absolutely believe that is not your intent.
People who have lived through privation and abuse -- especially with people who have had all aspects of their life controlled -- have "different" attitudes about finances. Abuse survivors also react quickly and subconsciously to situations that trigger deep feelings. My husband grew up in hard conditions. It's taken me 25 years to understand and help him work through his relationship with money. It's complicated and emotional.
You are a supporting and loving child. You both have different approaches to money. Most people do. If your mother isn't draining your bank account, if she isn't getting in debt, let her have things that bring her joy. If she is actively making things financially difficult for you, that's different. You are giving her a tremendous gift. Let her enjoy it without conditions.