Problem with intrusions

Hello!

Few months ago I had an intrusion into my wordpress blog. Because of that, I increased the strength of my key, also I

installed the Wordfence plugin in a high security level, and also installed Google Authenticator plugin.

Now I find in the list of reviews, in some few pages, signs that I have again suffered an intrusion, although apparently the page content has not been modified.
I am the only administrator on my page and each revision is recorded with my username and my custom Gravatar logo.

These intrusions are registered with a comma, not my username and the default Gravatar icon, not my own.
Wordfence never alerted me of these intrusions.

The first question is:
– This is a sure sign of an intrusion, or perhaps have been myself that by some reason I have been registered without

my name and without my own Gravatar?
– They have been brute force intrusions or is there another way to access the control panel of WP ?
– How I can prevent these intrusions ?

I appreciate the time that can be taken to respond, I am very worried with this problem.

Regards