I'm in exactly the same position with my 7 month old.
I took her to a chiropractor and she suggested I try taking some probiotics (I think that's what they're called). She gave me a powdered form and told me to take 1 teaspoon myself (I'm breastfeeding also) and give 1/2 a teaspoon to bubs before bed. IBS support tablets are the same sort of thing but the powdered form is better for some reason, sorry, I've forgotten why. My chiropractor checked and found her to be deficient in friendly gut bacteria..she didn't just pull the suggestion out of nowhere, so it might be a good idea to check with a health care professional before trying this..
Other suggestions I've had so far have been to go back to basics with the solids and just do pure fruit and veg, especially pears, prunes and pumpkin. So I've cut out all farex/pasta/rice/meat/teething rusks. I think it's helping.
Coloxyl drops and infants friend have also been suggested to me. The coloxyl drops didn't seem to help but for the past 48 hours I've been giving her infants friend and she's been going alot more often but it's still really tough and she cries each time. It's quite distressing isn't it?
Most people have suggested that given that prunes/pears/coloxyl drops and prune juice hasn't seemed to help, the best thing would be to have her checked out by a doctor.