Do you think it is her?

I've been broken up with my ex gf since last summer. Lately, I have been getting hang up calls at both my home and work. Someone has also been logging on to instant messenger with my id (i already changed the pword). Also, my ex is the oly one who knows my schedule at work where I get the hangup calls at almost the time I get in at both work and home. Do you think this is her? Why would she do this? It's happening a lot lately! And, yes, I still love her, but i just don't know if she is sane anymore?