In my >50 years of receiving (ex-wife and ex-GF's) or taking (me, solo) enemas, I've had what feels like 3 noticeable kinds of discomfort, but none of which rose to what I'd call pain.
1) cramps, felt almost anywhere,
2) intense urges to push, open sphincter or evacuate, usually felt very low in the rectum,
3) non-crampy colorectal wall irritation, felt anywhere, feeling like an overly strong solution (soap, glycerin, etc.) is just burning or irritating the wall.
Now there are many other enema feelings including water or gas movement, rectal temperature changes and internal mild to strong pressure and fullness which I enjoy, never painful at least so far up to 4-5 quarts of enema.
The only times I've felt like it was a bad enema were a few times with either strong soap or glycerin or both, which gave me such strong #2 pushing urges that I felt like I was leaking and couldn't control my sphincter, almost like I was about to dump the whole enema before I could reach the commode, even with a DBC inflated. Fortunately, the real accidents were limited to a brief spray just before getting to the commode seat.
I never try to intentionally produce cramps, but if they're pretty mild to moderate I can actually enjoy them as I know they're going to end shortly. Even up to my routine 4 quarts or rare 5 quart enemas held for 20-60 minutes, I've felt and seen very pleasant pressure and distention, but never pain or any hint of nausea. Now taking and holding that much volume and water weight makes me feel very full and distended, and rather awkward to even roll over or stand up and walk. But personally I find those pressure and distention features to be very pleasant and erotic, especially when being consensually forced to endure them by a trusted Domme giving it to me.
Eric