Trouble going to the bathroom.

First off i want to say i really wish i could do the paid option so i could for sure get the answer and so you would get paid for being kind enough to answer our questions but i just dont have the money,whitch is why i want to know first if its someting i need to bring her to a vet for. so my cat lillie when she goes to the bathroom when she gets out she dosnt walk very well on her bach legs kind of like shes having some sort of a sapasom. she walks find any other time just when she goes to the bathroom.she walks really weird for a minute lays down for a second and then is ok again. i can touch her back and stuff and it dosnt seem to bother her she plays ect. but once she goes she gets weird.she acts almost like somting is stuck!? and aslo i wanna add that it smells horrible not just a normal bathroom break.this has been going on well that i know of for about 3 weeks id say. she is a little chubbie but nothing to bad and she eats dry cat food somtimes wet for a treat.shes a indoor cat NEVER goes outside shes fixed so im not sure whats going on but i really want her to be ok. please answer my question, if you dont ill understand but i would mean alot to me. thank you