Yes, I see where Emanuel is going with line of questioning. Sometimes a low back injury can cause issues with the spinal cord which can then effect bowel and bladder issues. So it may have nothing to do with the core exercise and something to do with the spine, which is a red flag for immediate medical attention.

When you activate "core" the idea is to activate transverse abdominis and multifidis, but to complete the whole package the diaghragm and the pelvic floor get involved as well. The problem is with overactivation you may be developing pelvic floor trigger points which can cause issues with bladder. Usually it's so that you can't hold it or have urine release with coughing, sneezing, that sort of thing.

The not be able to go easily points to the red flag or some other issue you may need to get checked out by your doctor. Don't delay with urine retention. It can become a serious situation quickly.