blend air door?

ive been trying to search a little bit and ive kinda come to the conclusion of this. i replaced my heater core and after i did, the heat worked...too good . no matter if i turned it hot or cold, it just blows out all hot air. so ive kinda assumed that i did something to the air blend door? someone said that i mustve knocked this and messed something up. i really dont want to, but might have to take the whole dash out again to fix this. so please help me and give me ur opinions! and if u think its the blend door, advise me how to fix it once i get to it. thanks!

thanks for the responses guys. im working on checkin the vacuum lines but they all look fine... so if it is the air door, is it something i can just fix or would i have to buy a new one?