What Causes Painful Bowel Movements In An Infant?

Hello, my name is Brooke. My daughter is almost 3 months old. I m a vegetarian. She is breastfed. She is a gasy baby.. but recently she seems like she strains to poop and will cry. Tonight I couldn t calm her down, she wouldn t even take my boob..it was like she was in pain. I ended up giving her some prune juice with some breastmilk and she pooped ALOT and then she felt a lot better. Could there be a blockage or something going on? I ve read that now that she is getting older she will poop less, but if that is the case then why would she be crying when trying to poop and why would so much poop be in her diaper if she only has to go every so many days? She has gone almost two days without pooping and will push and push and only little farts will come out, but as soon as I give her the prune juice with breastmilk shortly after is when she fills her whole diaper. Is there something I can do.. should I start adding water to milk? I hate seeing her cry in pain.