You're starting too late in the enema. The first feelings are when you decide or are told you're getting one. If you're doing it yourself you feel the water as it gets warm and then fills the pitcher/bag/can/whatever, and the slippery soap if you are using it. Then the weight of the water as you hang up the bag. Then it gets more real, the feel of the fabric and then air as you your pants down and off. Then there's the feeling as you get into position, which usually includes the buttocks separating and a bit of cool air on your anus.
Then you hear the snap of the lube and see it go on the nozzle, glistening. If you're doing it yourself you can feel the thick slickness on your finger and the hard nozzle in your hand. Perhaps the nozzle has some of the enema water dripping down it from being "burped" and you feel that, cool now, as you smear the lube on. Then the feeling of the chilly gel on your anus and, if you're lucky, a finger circling, then penetrating to get you ready. Then the nozzle slides in.
After that you feel your body relaxing as you take deep breaths and get yourself ready, then you hear the click and feel the water, cooler at first then warming up, fill your rectum. For me there is often a period of strong urge. It's not a cramp, but I just feel VERY full and like I have to go RIGHT NOW! Shortly after that the bowel opens up and the enema goes higher and the urge goes down.
I can dimly feel the enema as it makes its way higher. Not in a way where you can point out right where it is, but you can tell as it flows around the flextures of the colon. The urge to go isn't very strong, and the cramps usually aren't there unless I'm really backed up, but there's a feeling that lets you know they will soon be there.
As I take the bag the volume taken up by the enema becomes more apparent. I feel my belly bulge out more and the weight of the water pressing on my insides. Some parts are erotic, others are odd... but I can feel the colon swelling and pressing on my insides.
Sometimes it gets to be too much and I don't get the full enema in, and I race for the clamp and shut it down. Sometimes I stay there and hope to just hold it a little longer, sometimes I'm fighting to hold it just until this cramp goes away and then it's an instant dash for the toilet. Usually not, though. Usually the bag empties and the enema slowly gets to work and makes its presence felt more and more as the minutes go on. Usually for me the urge starts first, with my bowels telling me that they really want to empty out more and more forcefully. Later the cramps gradually add to the crecendoing feelings, usually starting on my right side and going around to my left - at least at the best times. LisaK usually brings up her twist/bite rag that she has. I don't have that, but I do have a penis and I will tend to grab it as things heat up. Sometimes it is to masturbate, other times it is just to have something to squeeze and hold onto as a particularly intense feeling runs across my belly. What are the cramps like? They usually travel along my colon and do have a component of mild/moderate pain, sort of like your muscles get when you have been running a long time. Most people have experienced a similar feeling, either as "gas pains" or when your bowel is riled up from illness or eating something you really shouldn't have.
Finally when I let myself go I stand up, and usually that triggers a much more forceful round of urges and cramps from my bowels. I move to the toilet and quickly let it out. Sometimes everything happens immediately as a great cascade coming forth, other times I get somewhere between a pint and a quart out and then it stops for a while, but I feel it inside working and getting everything together*.
Lastly there is the feeling of relief that it's all out. This varies depending on how badly I needed the enema, but is usually there. The feeling of "lightness" that people describe. Then of course there is the look at the bag, water still beaded on parts usually, and the decision on whether to do another enema or not, with the knowledge that it will be much easier and more pleasurable this time.
*There are some times when things just aren't working at all, and the water sloshes around inside and comes out over hours in little squirts. I hate it when that happens.