Yellow spit up!

La

My son is two weeks old. He breastfeeds well. For the first time EVER he spit up, and it’s VERY yellow! He had just finished nursing.

Not sure if I should be concerned. I always burp him after he feeds and he doesn’t have a hard time

burping.

Any idea as to why this happened? What can I do? I feel like I won’t be able to sleep tonight, I’m afraid he’ll do it again and choke or something..