You mentioned you are under stess. Have you read Spengler's last post about how some people under stress tense up their pelvic muscles and cause themselves to feel that pressure in their rectum? It certainly wouldn't hurt you to try and de-stress, be that with medication or with yoga, tai chi, deep breathing, etc. techniques. Some people here are on an anti-depressant not so much for their mental health as for the fact that one particular brand actually helps IBS-D symptoms.
Most all of us on here do not recommend any sort of colon cleansing product. If you have frequent bouts of diarrhea, you are getting plenty cleaned out. Stuff just doesn't stick to your intestines like gum sticks to the bottom of your shoe; if you move regularly or have diarrhea, everything is coming out. When you are constipated, yes, you may need a laxative to help you out, but there's not need to "flush" your system with anything. You're either feeling pressure because you're constipated--which a regular, much cheaper laxative will fix--or you feel pressure because you are tense and/or because your intestines just send out that feeling, even though there's nothing there (I think that's part of "functional bowel distorder"). And a colon cleanser will not make your tension go away.
If you really want to poop yourself silly for a day, then get some Go-Lyghtly at the pharmacy. You can get it without a prescription and it doesn't cost but $4-$7 bucks. It's what a lot of people take prior to a colonoscopy. Tastes like you-know-what, but it will make you poop water after a few hours. At least by taking that it will 1) be cheaper than most colon cleanser products and 2) you know it's something doctors prescribe, so it's not going to harm you. Those herbal things are not regulated by the FDA. Could have bat guano or tiger testicles in it for all you know.
If you are intersted in a less chemical laxative, try either prunes or aloe vera (you can get aloe in pills so you don't have to drink it). People have been using prune juice to stay regular for years. One person on here has said eating prunes has helped her diarrhea, oddly enough. Perhaps the fiber from the actual prunes helps balance things out. My mother took aloe vera pills for hair loss, only to find out that they were making her a bit too regular, so she had to cut back to one pill every other day. Hopefully one of those things will not only help with your constipation, but moderate your diarrhea as well.
The problem with treating your diarrhea is that you can go back to being constipated so quickly. If you have diarrhea, you need to be extra careful with what you take so you don't swing back to constipation too roughly. Me, I can't take a whole Immodium unless there's something really bad wrong with me (like my episode back in the summer where I had diarrhea every day for two months); normally a whole Immodium pill leaves me very, very constipated the next day (before I had my gall bladder problems, I leaned towards being constipated). So if I have diarrhea I take half an Immodium or two Pepto Bismol caplets. The Pepto Bismol has never constipated me and that's what I used almost exclusively when I had diarrhea from my gall bladder. If I haven't gotten better after 30 minutes, I'll take another dose of Pepto. With Immodium, though, I wait an hour or more before I take the other half of a pill because I'm so afraid of making myself horribly constipated.
There are two other things I recommend for diarrhea, but if I were you, I probably wouldn't use them unless I was having a long-lasting period of diarrhea. One is calcium--particularly Caltrate 600 w/ vitamin D. That helps some to stop you up. Don't take more than one pill a day until you are absolutely certain that it's not going to constipate you. If you are still having diarrhea, then up it to two. The other thing is dehydrated banana chips. Those things will constipate anybody! (I'm still waiting, though, to see if Sarita has tried them; she's the hardest case out there). Never eat more than a handful a day and only when you have diarrhea. As far as I am aware, neither the banana chips or the calcium will offer immediate relief like Immodium or Pepto, but with regular use, they should eliminate the need for the pills all the time. Obviously don't use the two in combination.