How do I explain to my mom why I'm on PrEP?

Long story short:I got a prescription for PrEP (Truvada). My insurance is through my parents and I specifically told the pharmacy NOT to call about my prescription being ready, refills, etc. Well they did, and they left a message.I got a text from my mom the other night asking if I had a prescription (the pharmacy called for "insurance authorization"). I told her "no". Well she called back the next day and caught me in my lie. She made it clear to me that she knew I was lying. Maybe I shouldn't have lied, but I was afraid. I text her back and said "Don't worry. I'm okay. We'll talk about it later". She never replied.My mom is very conservative and often close-minded when it comes to situations like this. She doesn't even like the fact that I'm gay, but she's gotten better about accepting it over the years. I suppose I can't really blame her, given that's how she was raised.So how do I explain this to her? I have a feeling that it's going to turn into a fight and she's going to think I have HIV even when I tell her I don't. Has anybody been in a situation like this?Thanks