I agree with Timbales in the irritation factor. And its probably your timing. I actually suggest paying attention to your diet more or taking a fiber supplement so that you don't need to douche before bottoming.
There's actually a fiber supplement made for gay guys that I just started using and its great! Its called Pure for Men and it costs about as much as a fiber supplement from the grocery store, but I think its better. Its vegan and has chia, flax seed, and psyllium husks... most just have psyllium. A great product, especially if your diet is the reason behind the feeling that its necessary to douche before bottoming. Also reassurance for when you don't have the opportunity to douche before a spontaneous roll in the hay.
If you still feel the need to use an enema, it does irritate things down there. No way getting around that, but you can try and make it as gentle as possible. Make sure its normal, clean, pure water around the same temperature as your body so that you don't shock your insides. I like to take a hot shower, squat down and relax under the water until I feel hole open and relax, then douche. And the only place you should be cleaning is the lower area of your rectum which is normally empty unless you have to poop. If you force too much water in there... things get sloppy because you've forced water to where things aren't solid yet, and then pockets of water get trapped in the twists and turns and then that water finds its way out when you've got a cock up your ass. Not fun and very embarrassing, especially if the other guy is an asshole about it. Pick up one of those baby things they use to suck boogers out of infant's noses... that's about the amount you need to gently clean yourself.
Also timing of using an enema is important. Take a BM about an hour before you douche, and douche at least an hour before bottoming. Reason being when you take a dump, that opens everything up so if you douche, you're going to have the sloppy problem I mentioned before. Also your ass needs time to settle down after you douche. You've just forcefully pushed water inside yourself, when your body isn't turned on and ready to take anything inside of it. And you've taken the mucus membrane away which has basically dried you out and leaves you very susceptible to tearing. So if you give it some time, your body's natural defenses will have time to get back to where they should be.
But if you're bleeding with this top guy, is he being too rough, too quick? I've bled from a guy shoving a thick 10 incher in with barely any lube... I didn't know he was going to do that right away, and it was not fun, especially when there was blood in the toilet later on. Remember communication is important and YOU are in charge because its your body that he's poking around in. If something doesn't feel right, take the time to it figure out... don't worry about ruining a moment. It will be 10x more sexy and pleasurable if both parties are comfortable and feeling good. And add more lube about every 5 minutes, have him rim and finger you, have him massage you so you relax more. Maybe you can't take it that deep yet so you have to gently ease it in until you can comfortably take it. I also think that being on top is one of the best positions to start in for a bottom. You're in control and can really pay attention to your body and get things lubed and opened up. Once you feel great, you should be able to move on to other positions and comfortably enjoy!
Anyways... my answer is try fiber supplements, lube, communication with your partner, and more lube.