Yellow spit up?? :(

My son is 3 weeks old and has been spitting up really bad the past 2 days. We are breastfeeding and supplementing with similac formula for supplementation. I haven't been breastfeeding as much because I think he has thrush which we have an appt tomorrow for.

He just ate some formula and I burped him, he spit up then, a lot, and about 10 minutes later I laid him down and he spit this up. It is literally yellow. Any one seen this??

I posted this in FTM's too, just hoping for a response. Google was inconclusive. Lol.