I have experienced this more than a few times (once last week, actually) where I actually had to throw up during an enema/clean out session, although it has all been within the past six months to a year and it only happens when I go for a really long session at it.
Ideally, I would have 2-3 shorter sessions a few times a week, but time has not permitted lately (hell, ideally I would have a nice relaxing enema every day like I used to have the time for!) so I have found myself doing these marathon enema sessions about once a week that last about 5-6 hours total time and can consist of up to 20+ two quart bags full (not all at once, but one at a time and expelled one at a time before another one) and in a few of these I have gotten nauseous and thrown up. If and when that happens, I either stop the session and resume another day or stop for a while, recover and finish up later on.
I was concerned that it was too much water but I don't retain them very long (maybe the first 5-6 I will hold for up to but no more than 5-10 minutes each, and then any after that I don't retain but a minute or so) and I wasn't experiencing any of the other symptoms of taking in too much water. So I am wondering if I actually got water up past the colon somehow (I didn't think that was possible, and especially since I don't take another bag until I have expelled one) or if it was just pressure.
The only time I experience this is during long sessions, a normal session for me is about 2-3 hours just after a natural evacuation, taken slowly and consists of anywhere from 10-12 2 qt. bags taken in and expelled at a relaxed pace over that time one at a time, and I have never gotten nauseous from that kind of session, in fact I feel great after one of them. I guess the lesson is take your time, enjoy and don't try to do too much but rather listen to your body and enjoy the process. I just want to get cleaned out really good and enjoy it, not make myself sick!
I am curious, though, if there is a way to prevent water from going up from the colon into the small intestine, if this is in fact possible. I think that has actually happened to me a few times.