The guy I declined a hook up with is now taking a sudden interest??

Sounds like he is just curious about you and not really that much into you. When you declined a hook-up with him, that made you stand out from other girls and gave you "appeal" in his eyes. He may be the type of guy who likes the combination of a girl being easy to be with but not so easy to get. He likes a good challenge. You come across as something of a mystery to him, which is good. If you can keep it up, he'll admire you from a distance at least and it possibly could lead to more. Time will tell. Hold out for a lot more than a hook-up, tho.