12:18 PM PT -- TMZ spoke with a rep for the Evanston Police Department ... who tells us Kristin was initially stopped for speeding early this morning.
During the stop, the officer learned Kristin has been living in the state and driving without a valid IL license, which is against the law in IL.
Cops say Kristin was required to travel to the police station, in her own car, and was ultimately hit with a citation for $120 ... despite the fact that EVERYONE ON THE PLANET knows she splits her time between L.A. and Chicago because she's married to Bears QB Jay Cutler.
Cops say if Kristin wishes to continue driving in the state, she must get a valid IL driver's license.
Kristin Cavallari says she was taken into police custody in Illinois this morning -- and forced to post bail -- and claims it all has to do with her driver's license.
KC went to Twitter moments ago and said, "Who knew having a California driver's license in Illinois was a CRIME to be escorted to the police station to post bail?!! What a morning."
The former MTV star -- who's married to Chicago Bears QB Jay Cutler -- is insinuating that she had the same problem Justin Bieber had in L.A. ... when he was cited for not having a valid driver's license.
In Illinois, you are required to have an IL driver's license once you have lived in the state for 90 days ... or you can be cited.
However, KC says she's not a full-time IL resident and therefore shouldn't be subjected to that law. She also tweeted about it saying, "I still have an apartment in LA so I'm a resident of California."
Story developing ...