Supplementing with formula @ 6 months

I had a scary moment yesterday. My LO has had small amounts (a few spoonfuls here and there) of both cows milk and soy formulas over the months and it seemed like soy irritated her less. I bought a different brand of soy yesterday (Enfamil) to try and I gave her some in a sippy cup. Just a small amount. She seemed okay, hours went by and I nursed her twice. Three hours after giving her the soy she threw up everything!!!! Ounces upon ounces. This went on for an hour..... She eventually stopped heaving and I was able to nurse her before bed and she was fine all night. Now I'm concerned about introducing any formula ever again. I don't know if it's the soy, the brand, or if she is going to have this reaction every time I give her any significant amount of formula. Anybody ever hear of this happening??
BTW, I know breast milk is best, etc. but I started back at work and am not pumping enough so just want to be able to give a bottle of formula a few times a week to fill the gap.