Newborn diarrhea?

Hi everyone, my son is almost 3 weeks old. His first week and a half he was EBF, but then we switched to formula. He has been using sensitive formula as that was what his older brother used and we just bought the familiar one. Since starting the formula he had about 5 days of constipation but he was still pooping about 3 times a day just hard balls, however for the last 3 days or so he has had diarrhea. No fever, no signs of dehydration, just constant diarrhea. He pooped over 14 very full, watery, poopy diapers yesterday. Its yellow-green color and so liquidy that it soaks into his diapers like pee. He also pukes fairly often for a newborn, at least puking up one full feeding a day in addition to spitting up frequently no matter how much we burp him or keep him upright. I'm keeping a close eye to make sure he doesnt show any signs of dehydration, but does anyone know what this could be? I'm thinking of trying out a soy formula as maybe a milk allergy? My first never had issues like this so I'm lost.