My DS was 6 weeks early so I couldn't nurse for a while. I exclusively pumped for a while and tried to nurse when he was able. After two months my supply wasn't enough to keep up with him and he was too lazy/stubborn to nurse so we had to switch to formula.
We have been using Similac Advanced for just over a week now. I have noticed he spits up more also. His pediatrician said it could just be a part of him getting used to the transition between bm & formula. She said its important not to switch formulas a lot - keep LO on one kind for a while to give LO time to adjust.
How long has your LO been taking some formula? She may just need a chance to adapt.