opened emails in-box disappearing.

I am having issues with opened emails in my in-box disappearing. They are not in my trash or spam folders. They are just gone. no trance of them. Anyone having the same issue? 

Doesn't matter what the email is, once it has been open for a couple of mins, the email just deletes from my in-box.

My drive usage is @ 33%. I have been deleting old emails, getting the storage down with hopes this helps the problem, but no luck as of yet.