Hi Robin,
I did experience increased frequency with little/dribble at times after my first TURBT and before BCG treatments. My urologists attributed that to bladder spasms primarily because the nerves in the bladder were still recovering from the surgery. In my case we waited for about 8 weeks after my first TURBT before receiving BCG. Not everyone experiences the same, so I understand, as in my case I did not have the severe pain due to constipation as you did.
As to self catheterizing to determine urine retention, I have no “zero” medical background so my advice on that would be to reconnect with your Urologist for guidance. Sorry, I could not be of more help on this one.
Be strong and stay positive…God Bless!
Rick