help??

my 2 month old was first on the regular yellow enfamil because she was very gassy, she'd strain a lot, and would only poop once a day or go 2 days at the most with no poop, then we switched her to gentlease but she's still straining a lot, ONLY HAS very watery dark brown poop (diarrhea) and she's still gassy. her pediatrician gave me samples of the soy formula but i looked into it and seen a lot of people say to just use that as a last resort and keep trying other formulas. so can anyone recommend any formulas for those problems?