There are any number of reasons why cramping might occur. You don't say much about your enema habits so I'll just throw out some general ideas. I'm sure other members will chime in with even better ones.
Basically, when you take an enema, you're going in through the out door, so to speak. Your body may be saying "Whoa there, cowboy, that doesn't go there". We know otherwise, but that's what your intestines might be saying.
Water temperature is a common culprit. Cool enemas may cause or increase cramping. You should check the temperature of the water the same way you do a baby's bottle, with your wrist or the inside of you forearm. If it's comfortable, it's fine for an enema. Some of us like it a bit warmer, I know I do, but you have to be careful. Poached intestines are not a good thing.
Do you use any additives/irritants/purgatives like soap or liquid glycerin? All of those increase the amount of water in the intestines and also increase peristaltic action, causing cramps. For many enema users, this urgency is a desirable side effect.
You might be filling too quickly. Slow the flow rate down, fill more slowly and see if that doesn't help. Stopping the flow and rubbing your tummy until the cramp subsides is a good trick too. Also, what position do you assume when you take your enema? Some positions are better than others when it comes to discomfort. Some people recommend starting on your left side for 1/3 of the enema, rolling on your back for the next 1/3, and then onto your right side for the final 1/3 of the bag. You don't say what volume you take but it may be that you're asking your colon to take more than it can tolerate, you have to work up to these things gradually.
One, or a combination of all these things should help to relieve your troubles.
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LadyDoc