A couple of people have described a sensation of needing to defecate or being constipated or having diarrhea. This is a phenomenon known as phantom rectum. It is similar to phantom pain from amputated limbs. Sometimes the nerves that were once connected to your sigmoid colon, and especially your rectum, do not yet know that they are not connected to anything anymore. So, when you experience pressure from a colon full of fecal matter, your body misinterprets it as a full rectum and makes you feel like you did when you were constipated before you had an ostomy. It can make sitting quite uncomfortable. Some people find that actually sitting on a toilet, as if they were going to defecate the way they did before they lost some guts, releases the sensation temporarily because it tricks the body into thinking that you have dealt with the problem. But if there is significant fecal matter backed up inside, it won't relieve it for long (if you get any relief at all).
I have an ileostomy and even still, with the opiates that I had to take after my surgery until I weaned off of them, I did experience the phenomenon you describe. My heart goes out to you, dear. That is the best advice that I could give you. I even started taking magnesium citrate, which is, as you may know, about prep regularly just to have it be as cleaned out as possible to avoid the sensation. Now, I don't recommend that. Someone with a colostomy can find diarrhea awfully difficult to deal with as well. But even diarrhea can cause that sensation because it is the same sensation as diarrhea you felt before you had your ostomy. The best advice is regulating your stools by trying Metamucil. If you don't like the type you add water to, they make little crackers now too.
After falling and breaking six ribs in my back, I was on some pretty significant opiate pain control and it backed me up pretty significantly. So, I recently have been living this one again. Oh joy! Just so that you're aware, I hope this helps.
