What could be causing these texture changes that are more noticeable when Botox wearing off? (Photos)

I received 25 u. of Botox in my glabella region 3 mo ago. I’ve gotten Botox in this area for ~4 years. In certain lighting I’ve noticed a change in texture, and “waves” in this area. It’s more noticeable once the botox is worn off. It's not noticeable in bright light, or when looking straight on - only when my face is lit from the side. Could this be from being under-treated, treated in the same area too often, or from being a side sleeper? Or is it just wrinkles reforming?