[Help] my dog won’t let dogs sniff her butt

Help!

My dog has been very friendly and social her whole life (1.5 years). In the past month or so, if any stranger male dog tries to sniff her butt, she screams and snaps at them. She has always been the sweetest kindest dog in the world but now she will absolutely not let a strange dog come near her.

I feel awful going to the dog park because these dogs are just doing the most common thing that dogs do. Does anyone have any clue what could be causing this, and is there any way to train to fix it? She’s extremely trainable but idk what to do with this.

Side note: I haven’t done any tests but have spoken to my vet about it. We don’t think it’s a health issue or she would be upset with all dogs, and she hasn’t shown any other signs whatsoever of it being health related

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