I'm worried...

My son is a little over 3 weeks. He last ate 6 ounces at around 12:30 am. I tried feeding him at 3 am, but he threw up so I put him back to bed. My bf woke up around 6 am to go to work, so I tried feeding him again. He pretty much didn't eat since 12:30 am, and when I tried feeding him about 2 hours ago he only had 2 ounces. Should I be worried?

I've tried waking him up to eat, but after two ounces he was not having it.

How did he go from eating 6 ounces back to 2?