Need some advice ASAP please

Okay Momma's my baby was born premature and has been on Enfamil Enfacare 22 calorie ready to use formula since she has been born. We recently got accepted to WIC as I am a stay at home mom and we can use all the support we can right now, well they don't cover ready to use formula. We had to get a referral from my daughters pediatrician for WIC to cover her special formula designed to help her growth. My daughter has had a regular BM every morning/mid afternoon for months and it's been soft and enough to cause some blow outs! Well, I started her on 2 bottles of the powder a day 2 days ago until we ran out of the ready to use bottles and got to the point where she's on all powder. Since she started, her last BM was Monday the day we started it and I noticed it was solid and harder for her to get out. She didn't have a BM Tuesday at all, so today (Wednesday) I gave her an ounce of prune apple juice to help her constipation as directed by her pediatrician. About an hour after she drank it she started screaming in pain, she had 2 hard blocks come out and that was all. Then about 30-45 minutes later the screaming continued but WORSE. I tried helping her get it out and out came a wide hard tootsie roll looking BM with blood around it from her straining to hard to push it out. So my question is, why is the powder of the enfacare causing her to have solid BM but the Ready to go enfacare isn't? I'm very concerned and really don't know what I should do. I'm going to call the pediatrician in the morning but I'm worried about going about the rest of the night. She's 6 1/2 months old :( Any advice?