My leopard gecko is licking her bottom. Is this bad?

I picked her up and gave her a little inspection, and everything looked normal. I have read that it could be because of the substrate, so I am going to switch her to a repti-rug tomorrow, but I want to know what the other problems could be. She has never done this, so it was a little surprising. Since the... show more I picked her up and gave her a little inspection, and everything looked normal. I have read that it could be because of the substrate, so I am going to switch her to a repti-rug tomorrow, but I want to know what the other problems could be. She has never done this, so it was a little surprising. Since the outside looks fine, I want to make sure there is nothing wrong with her insides. Could someone please give me some more insight, and signs that something else might be wrong? Thanks!

(Always has fresh water, humidity around 25-50% on average, ~90 degrees)
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