wp-lastweets plugin was automatically installed

Hi,
I have a website (not live yet) that I am the only user of(no other admins, users etc). I recently noticed that suddenly a plugin was installed (but not activated) that I had not installed. I changed all passowrds (FTP and backend) and deleted it.
Just now I noticed again another plugin automatically installed, this time it is the wp-lastweets. Just installed, not activated.

Please, can someone tell me how this can happen? Or are there other plugins that will randomly install this plugin at some point? I am very worried.

Thanks!