Milk formula

My LO is 2months and have been drinking Similac Infant stage 1 since birth. We notice that she have difficulty pooping (every 2-3 days, she will poop once) and gets very gassy (sometimes it is difficult for her to pass gas and will cry). Since 4 days ago, she started to vomit at least 1h after feeding even after burping. We feed her 120ml, previously she will drink it all but now she always leave behind 30ml (drinks 90ml only). Her abdominal will be hard after every feed and she will cry loud. This only happens during the day feeding. When it is night feeding baby seems ok. Is this formula not suitable for my baby? I thought of going for soy-based formula. Is it ok?