Mike,
It's possible that your lactose intolerance is affecting your bowel movements, my daughter has lactose intolerance and has similar problems. The other issue you're having about cathing and not emptying your bladder, etc., I have a little experience I will share with you. First of all I hadn't needed to cath until recently (I had RC and neobladder surgery 2/04), but let me tell you that you may still have a considerable amount of abdominal swelling from your surgery and this might be part of why you're not noticing that full feeling. For perhaps the past one plus years I get a full feeling in my lower abdomen, more on one side than the other (my neobladder is mostly in the right side of my pelvis). So you probably need to give yourself more time for your body to be back closer to normal for you, and try not to worry too much.
You mention that by the third or fourth (I'm assuming you mean the loose ones) movement it gets quite painful, you might want to find a product I use for cleaning the rectal area. Now don't laugh, because it's Playtex Personal Cleansing Cloths and they are wonderful. The best part is they don't have any alcohol in them, and you can flush them after you use them. You can get them at Wal Mart or any drug store and they'd be in the Ladies Personal Hygiene area, they come in a plastic container or you can get them in a resealable refill pack. I get the resealable pack only, no need for the dispenser. You could probably find them near the products that are used for people with incontinence, like pads and adult diapers, as well.
I hope some of this helps you in some way, and keep us up to date on how you're doing.
Lou Graham