First, if she is sore and weak, her listlessness and urinating where she lies isn't unexpected. She probably couldn't face the trek to her box. Therefore, you may want to keep her on a towel or something absorbent for ease of cleaning her up should she need to go again.
Now when cats suddenly develop these signs, we do have a range of concerns. Since her limbs are not cold and she does have voluntary movement, we'd be less worried about a full clot blocking the vessels to her back legs and spinal paralysis is also less likely. Therefore, if she had any earlier access to outside, trauma would be a major consideration. Otherwise, a partial clot, compression of the spine (from a disc protruding, tumor, abscess, blood clot, or swelling), or issues affecting the brain (ie tumors, bleeds, swellings, diffuse infections or toxin build up secondary to kidney
or liver issues) would be our other concerns. Just to note, arthritis is also a concern but usually arises as a slowly progressive issue and not a sudden one as you are seeing here.
So, these would be our concerns for her. If she has been drinking more lately, then issues with her organs and blood pressure may be the underlying cause for her sudden deterioration. And if she has weight loss, this supports a chronic issue that is now coming to a head as opposed to a sudden one.
With all this in mind, since she is crying and potentially sore, we'd want to have her seen urgently. The vet can start her on pain relief by injection to help calm her and get her comfortable. Afterwards, they can examine her and perform a neurological exam to determine which of the above is a concern. If organ issues are suspect, a blood sample can be tested to determine this. Depending on their findings, they can advise you if this is something we can treat to help get her comfortable and back on her feet.
Please take care & do let me know if you need any more information,