I think u know the reason because u have mentioned it in this post.

'you broke up with him' and then u just didnt expect him to move on so quickly. I think that might be worrying u. But u need to say this to yourself time and time again that u wanted this. U weren't happy with him. Well, some people heal quickly and others dont. Instead of worrying about him try to focus on other important issues

i hope it helps.