I know you want to know about probiotics, but why are you having to do this all on your own?
Since she has a "history" of these issues, has she ever seen a pediatric gastroenterologist for a full and formal work-up and tests? Or have you just dealt with the rather unhelpful pediatrician and then been left to figure things out alone?
While it is indeed good that you are thinking about her diet and making connections on your own, you should also have solid professional advice to go on. I would tell -- not ask, tell -- the pediatrician today that he or she will give you a referral to a pediatric gastroenterologist. You may have to get pretty assertive, or if needed, downright insistent. The pediatrician should have done it a long time ago, from what you describe. Seeing the specialist is not just about finding out what IS going on; it's also about ruling out more serious things so you and the specialist can really pinpoint the issue.
Your kid's suffering and has a history of suffering. I'd get the referral and then once it's done, dump the pediatrician ASAP for one who actually cares. You are doing great as a parent to be thinking through what might be affecting your child. Now MAKE the medical professionals help you. It steams me that the pediatrician is so cavalier about your child's health.