Why does it hurt so bad after having my crown reattached?

If you don’t have a root canal done probably you need one after do the crown. I recommended you go to your dentist to check it.  

It may be that the crown didn't seat all the way down and now you're biting on it harder than you used to. If that's the case, it may need to be adjusted or redone. There's also always a chance of a tooth needing a root canal after any treatment. If the nerve of the tooth was irritated while the crown was off or during recementation of the crown, sometimes that will calm down. But sometimes it won't and you'll need a root canal. I can't say what's really happening without an exam and xrays.

Hello
It´s depend on the teeth, most times it is only sensitivity, or poor performance of the procedure, please must go to the specialist to check it.
Regards.