My baby is 3 weeks today and he hasn't eaten much.. He ate last night at 1 am Then again at 5:30 am…

My baby is 3 weeks today and he hasn't eaten much..

He ate last night at 1 am

Then again at 5:30 am

But since then he doesn't want to eat. He's woken up crying like he is in pain and starts gagging like he wants to throw up but he doesn't and he falls right back to sleep.

He usually eats every two hours so this really concerns me :/

I called his doctor and they said they'll have someone call me back. But I'm not sure how long that will be...

Does anyone know what might be going on?