What does it mean when a guy doesnt call when he says he will?

my bf and i have been together for a few months and we have been great up untill lately, hes made a hbit of not calling when he says he will and when he does call i feel like he isnt listening,we used to laugh at every little thing, but now we spend more time arguing, we used to be able to talk for hours, but now we can barly think of enough to talk about for a few minutes, I used to be able to trust him with my life, but now i just dont know, im worried that we're losing what we had and i dont know how to get it back. please help.