Constipation Ease for 3 Month Old

So my 3 month old has been switched for soy formula because of tummy issues. She can't handle regular formula. But it has stopped her up like crazy. She has tried to use the bathroom twice today and she grunts and grunts and when she's finished and I check her diaper, there's merely a small pebble. She won't drink prune juice, I've already tried that. I bought some of Mommy's Bliss Constipation Ease but then I read that it was only for 6months+. Can I just give her a smaller dose or should I try something else?? Any help would be greatly appreciated. Thank you!!!