Girl, the same thing happened to me. My ex NEVER visited me at work before and demanded to see me one day. My boss was very concerned because the ex was being verbally aggressive. I told the boss 15 mins and he'll be gone. So I clocked out and gave him hell. He just had to have both the floozy and me. He was doing her but still interacting with me. We didn't have kids but we had a lot of history. I got his townhouse three more months rent free all to myself just becuase of all the guilt he had over leaving me. I didn't move out until I found a place that allowed my 2 dogs. So imagine my surprise at him wanting to see me at work. I was never home at our 'old place' when I knew he would be trying to come see me.
We were finished but he kept wanting to see me. So that's probably why the GF is harassing you. She knows your ex wants to see you, hang with you etc. She does not like that. She is feeling threatened by your history. Too bad.
It was when I made good on my threats on the boat we both had together and his job reputation, all things ceased between us. We do not talk anymore. I also chewed him out on the phone over the harassing call his ex GF made to my work. I was so mad that the whole kitchen heard me. He muzzled his GF pronto. She never bothered my work again. However, when I was at at the old place about the boat and walked away after that---when she found out why I was there (I spoke fast enough that only he would understand me at the door) she flew out the door after my back but my ex grabbed her just in time. I felt her hand down my sweater so I issued a second threat without bothering to look at his face, he can understand me without looking at me---SHE's not to hit me ever, I know about her assault record---while he was using a MP hold on her while I walked away to my car. We never saw each other again. I heard that they were not together anymore. HA!