There was a mid may update. Unless you are cannibalizing your internal links or not adding any value. Better to do link audits to make sure you are not using the same anchor text for the internal links. If you are using WordPress, you can check out Linkilo, when it launches. Otherwise, use Ahref or other tools to see every anchor text you use.

But I can 99.9% say the reason is a mid-may update.