The messenger module does not update the database.
I do not know what is going on with your site. It sounds like possibly someone has altered your database or installed custom code, but I have no way of knowing.
I would make sure you are not allowing any strangers any edit access on your site.
I would check to make sure there is only one file in the folder:
and also make sure there is only one file in the folder:
I would also look around in the database for any triggers that someone may have added
I would check that no-one except you has administrator role
I would change my ftp password
I would use Administration > Security Advisor to make sure your site is configured securely
If you don't know how to do these things I would suggest get a more technical friend to help or hire someone with more technical skills to investigate it.
Possibly its just automatic form populating happening by your browser preferences updating form fields based on previous forms. ie your browser is filling in the form for you automatically based on previous forms and when you save it that is when it updates the database, the browser doesn't update the database it just updates the page, but as soon as you save it updates the database.
How to disable form autocomplete in Firefox
How to disable form auto complete in Chrome
Hope that helps,