Ex is being ridiculous?

We had a bad break up and I vowed to never speak to him again etc. Then after our fallout he started to come over to my department (we work together) to talk with other coworkers in front of me. I'm not going to lie it got me real upset and he saw that so he started coming to my area a lot more and I felt that all he was trying to do was get a reaction out of me. He even occasionally would try and talk to me about work by asking unimportant stuff to which I'd answer short and mean. Anyway, long story short, I ended up going to HIS department to file a document. I felt like maybe he was implying get over it hence him coming to my area like nothing. So I went to file the document (he sits next to filing cabinet). Shortly after he bulged in my department pissed (I could sense it when he told my boss something work related and saw his demeanor had changed from earlier). Now he totally avoids coming to my department and earlier I walked to the common area at our business and he saw me coming while he was walking out his department door and quickly walked back. I don't understand why he's being this way? It's ok for him to come to my department for the stupidest reasons but when I have to actually go around him for business purposes he gets upset? Why is he being such a brat? It was ok for him to come around me but not ok for me to go around him and his work area? Why is this?
Ex is being ridiculous?
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