Why is the ac in my expedition blowing hot air?

I have 2000 Ford Expedition and recently my ac stopped blowing cold air. It will blow cool air when I first start it, but then goes straight to heat. I've checked the refrigerant and its full. The compressor is also working fine. I changed the relay to see if that would help but no change. I don't know what it could be. Any thoughts?

Also the heater core went out about a month ago. But still had cold air until about a week ago. Don't know if that would be related.

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