Three weeks sounds about right for all the pain killers to have worn off and for the healing process to have started in ernest. It may seem strange that the pain is only just beginning to mainfest itself but the intestines and colon are part of the autonomic nervous system, which does not have the same sort of connections to the brain as the nerves that nornally tell us we have pain elsewhere. This is why there is no feeling in the actual stoma but there is feeling from the surrounding tissue. Sometimes we don't feel the pain until the scar tissue reaches the part of the body that carries the nerve endings which signal there is pain.
I would reiterate what I said in your previous post and that is: Most of us are not medics on this site and if you are concerned about anything at all, have a chat to your stoma nurse to see what they have to say about it. I know that you have only just talked to her about the other issue but she will not mind if you come back with another question, especially as it is to do with pain. These nurses are there to help and are only too happy to share their experience and expertise. It is good that you can come on here and get an instant response but it will feel even better to have that response from a professional expert. The great advantage inappraoching them is that if your pain needs further investigation, they are in a good position to make arrangements to have that happen.
Bare in mind that people like me are only offering an amateur opinion that may or may not have validity. If what I say turns out to be wrong, I will feel terrible about it if you did not get proper advice from someone who is paid to know about these things. So if you are reluctant to speak to the stoma nurse yourself, perhaps you will do it for us so that you can feedback what she says. That's always very useful to everyone.