I have some ideas, but really, no I don't know.
I remember having problems shortly after getting immunizations for school, seems like measles, I have always thought there was more to that.
I suspect genetics more than anything else.
I have 2 sisters, they both had babies last year, both girls, one week apart.
One sister, who has a child already with no problems, had a baby with severe problems, I think it is called osteogenesis.
The other sister, had a healthy baby.
Watching these girls grow up, it is pretty much in your face, just how different they are. One is walking around into everything, wow she is a handful. The other cannot even crawl around yet.
Also, My mom was dx with IBS a couple years ago, but it's very mild, she takes no meds for it.