broke up with my ex about 3 weeks ago and he's already hitting on other girls. WHY???

[bee]

well for starters..im 18,(girl) and I recently broke up with my now ex boyfriend of 21yrs of age. we had been together for a little over five months. and don't get me wrong, he is an absolute amazing person. and before the end of our relationship he always treated me like a princess, and did things for me that he never did for anyone. and respected me in every aspect. but I don't understand how he can suddenly in the short amount of time start to hit on other girls. well to me is what I consider hitting on them.. for insistence comments on some girl's instagram pictures says that they look really pretty and such. and before our relationship started. he would write that on my pictures.(and that how I got the hint that he liked me) and during our relationship as well. and I know obviously he's not going to write that on my pictures anymore. or now he posts pics of certain things and hashtags them #hubbymaterial #greatfuturehusbandsomeday and stuff like that and it really bugs the crap out of me. maybe weren't meant to be together but damn its only been like 3 weeks!!! how can suddenly be completely over the relationship and moving on. while im still hurt and dueling. I wouldn't do that to him. and after all I actually started dating him because I thought he would be more mature about things but it really seems more childish than I expected. I feel like all the stuff he's doing is just making the breakup process longer for me, because I still truly love him with all my heart ( and im really not into that cheesy corny stuff) but I really still love him and its not like I can turn the love switch and say that I don't.
for me I seriously have no intention of flirting or seeking elsewhere for quite some time from now. is he seriously already over me that quickly???



(btw im not one of those stalker ex girlfriends.. all of the stuff I see is either on my facebook feed and my instagram feedback.)