I am certainly in no position to say "Yes, the fall out of bed was the cause!" That can only be done through careful analysis, guesswork, and memory. i.e.: Did he start changing directly afterwards or long afterwards. Then you have to figure in the different levels of normal cognizance and behavior for the time you're analyzing. For instance, an infant will have shorter focusing ability than a typical 18-month-old. An 18-month-old is more capable of acting out physically than a younger child. Thus, it can be really difficult to see when the symptoms actually started to manifest themselves.
I teach kids with different disabilities and many of them are supposedly ADHD. One of them was diagnosed a few years after a dog bit him in the head at the age of 3. A small part of his brain was removed. The ADHD symptoms could be attributed to that incident, but later on, it was discovered that his older brother has the same "disorder" and now that I know the mother, she's certainly a huge candidate for it.
I have a couple of other students whose parents seem relatively calm and organized, but their child can't focus, concentrate, and is often jittery. Who knows? I feel that way a lot myself, but am not ADD or ADHD. My parents could have sworn I was a hyperactive nutbag when I was young though! :)
I've not given you any answers, and may have opened up more questions. I just want to tell you that you shouldn't feel guilty for anything you ate or drank (as long as you weren't smoking and boozing it up!) while pregnant. It's completely impossible to do everything perfectly, no matter the importance. You sound like you love your son very much and that's what is important. There's a lot to learn about ADHD or similar symptoms, and A LOT you can do. This site seems to mention some very good resources.