Breast milk to formula... now vomiting

It’s currently 2am on a Saturday so if I called the pediatrician to ask I would be sent to the emergency nurse so I figured if it could be normal I would ask here first...

My baby is 6 weeks and is drinking breastmilk, mostly out of a bottle because we have some problems breastfeeding. I decided to try formula because this has been a difficult process for us and I just wanted to see if it would work. He’s had formula in the past and today I mixed half breast milk and half of the formula he has had before and a few hours later he vomited and he’s vomited twice since. Two times it was white and clear like milk but one time it was yellow and mucusy. Seeing him vomit, I decided I’m not formula feeding and I am going to continue pumping breastmilk for him because it broke my heart... but when is vomiting considered a trip to the doctor or urgent? He’s acting totally fine. But i gave him a bottle of breastmilk and he just threw it up... what do I do?!😭😭😭