Changed baby formula (cow's milk) at 6 weeks due to constipation. but now the soy protein based formula also constipates her (shes now 3 months). she cries when passing stool. her stool is s greenish in colour and the texture is quite hard.

Infant hard stools: are not uncommon. Although it is recommended to avoid supplementary foods until 4-6 months of age, often feeding a half oz of prune juice mixed with equal parts water will provide enough fiber and stimulant to remedy the problem and should not interfere with normal advancement of foods.

Define "constipated": If the stool is soft when they go it is not constipation. Greenish poop usually means it went thru sooner than it would if it were yellow or brown. Straining/fussing during pooping is normal as they use their breathing muscle to aid weak belly muscles & generate pressure to go. If the poop is hard full strength prune juice 1 oz 1-3X/d usually gets them looser in 3 days.