I need help understanding why my ex keeps looking at me ?

i was with my ex for over a year, we had many ups and downs over this time but one month ago he said things were finished totally out of the blue, as we work together i knew this would be awkward but for the first two weeks he totally ignored me walked out of rooms etc when i walked in. Then over night he suddenly changed to following me into room needing computers were i was when there were free ones else where, hovering around my working area and staring at me all the time. I have left that work place because i had already put my notice in as i was starting a new job so we could get money together to get a house together, and don’t really understand why he changed over night when i was in my last week where he wouldn’t have to see me again anyways. what does this mean alot of people say he is regretting his decision are they right?