A lot of doctors can't diagnose it either, lol.
I have to admit, I'm one of those people who only goes to the doctor when I get really sick and absolutely have to have medicine, and I have little to no faith left in GIs. Too many horror stories on here, not to mention that I was dismissed has having IBS for five years when, oops, it turned out to be my gall bladder all along. The last GI I went to did a test for occult blood, without telling me she was doing it and without telling me the results were negative, and tried to schedule me for a colonoscopy, although my symptoms didn't make me a candidate for pretty much the only thing that shows up on a colonoscopy: inflammation. Luckily I bowed out of that test, and after finding this board and calcium, I was right as rain again.
If I can manage to survive by self-medicating and not seeing a doctor and not having a bunch of tests run, I'll do it. I've been through my round of tests that proved nothing, so I don't care to do them all over again. I strongly suspect you will be told the same thing most everyone else is told: sounds like IBS, get more fiber. If you're not lucky, you'll be put through some mildly invasive tests which will prove that there's nothing physically wrong with you--hence you have IBS--and at the end of the day you'll be short a good deal of money and still have nothing to show for it but that glass of orange Metamuceil staring you in the face.
Going for several days without a bowel movement, especially after using a chemical laxative or enema, or only ever being able to achieve a bowel movement by using chemical laxatives or enemas, is very great cause for concern and you should see a doctor in those cases always. There are some medicines out now to treat chronic constipation, but they appear to be very strong, and only for people who can't manage to have a bowel movement otherwise (anyone approaching normal usually gets bad diarrhea from them). Have you tried treating your constipation with fiber, prune juice and/or aloe vera yet to see if you can make yourself regular? Several people swear by walking/running daily to keep them regular too. If you can manage to make a bowel movement, but it's not very pleasant to pass, I would suggest upping your fiber and trying non-chemical laxatives first. If that doesn't help, then when you do see your doctor, and he starts in on the "get more fiber" schpeal, you can tell him you already get the maximum recommended dose a day and it's not helping, can you explore prescription options?
Also, look into probiotics, like Digestive Advanatage (found in chain pharamacies in their bowel products section). They, like fiber, can cause a little extra gas initially, but many people with either diarrhea or constipation have found that they help their bowel movements and/or their gas and pain problem in the long run. You can also try a chewable pill called Equalactin (also found in the bowel products section). It contains a type of calcium that balances the water in your digestive tract and helps moisten your stools if they are too dry (i.e. constipation). It's pricey for long-term use ($4 a box, which lasts anywhere from a week to a few days), but I certainly saw better bowel movements from taking it.