Thank you. I have concidered this as well, however the effects are quite different, at least for myself to warant it only being from the mucsle tensing, however if it was from some type of toxin, then the change in stiffness after urination is also quite rapid. Thank you for confirming that this is the situation for you though. I have also had several bowel conditions, I don't know if you can distinguish, but when having to hold my bowels, which requires even greater muscle tensing, the effects, espcially of the extreme stiffness and immobility are not the same. HAve you ever noticed this as well? I am not sure the urine is directly the cause either, but it could be somehow related to the kidney or bladder.
Thank you again for your comments
My guess is that it's because when you have a full bladder and are "holding it" because you can't get to a bathroom, you are tensing muscles, even if doing so unconsciously. Tensing one set of muscles can inadvertently cause you to tense other sets of muscles so it can be a roller coaster effect. I do notice that when I REALLY have to go and have to wait for one reason or another that my entire body gets tense (and yes, that does increase my pain levels).
I don't think it has anything to do with the actual urine though.