We're having formula troubles here too. My LO has been on a mix of Similac Advance and breast milk since about 4 weeks (she's not 12 weeks). About a week and a half ago, she started crying and screaming at bed time with a stomach ache, which is very out of the ordinary for her. Then came the exorcist style projectile spit up, too.
Her doc suggested Gerber Gentle bc she might be sensitive to milk proteins. We thought that was the new perfect fit, but on day 3 she got the worst gas ever and was in so much pain while passing gas. We traded one problem for another. Doc now suggests Similac Sensitive. We're trying that and she's better today, but I know we need to give it a few days or weeks to see if it'll really be better. Otherwise, they said soy could be next.
It's a trial and error process and every baby is different! You just have to keep investigating and talking to your doctor until you find what works for your LO - that's what I'm telling myself, at least. Good luck!