CRJ200 First Officer reports a strong unknown odor after the flaps are retracted; followed shortly by a Smoke Toilet caution message. The Flight Attendant reports that the cabin is filing up with smoke. An emergency is declared and the First Officer applies left rudder when none is called for; causing a fuel imbalance caution and uncoordinated flight. Flight returns to departure airport as the smoke clears of its' own accord.

Narrative

During [our] initial climb out; after the 'flaps up; [after] setting climb thrust call; the Captain and myself started smelling a strong unknown odor. Not a few seconds later the 'smoke toilet' caution message appeared. The Captain had begun finishing the After Takeoff Checklist and pulling out the Manual to start the non-normal checklist for the 'smoke toilet' message. Not a few seconds later; our Flight Attendant calls us using the emergency call switch. She tells us that the cabin is filing up with smoke. The Captain and I immediately donned our O2 masks and the Captain declared an emergency. ATC clears us to come around and land. At the same time the Captain notices that we have a Fuel imbalance caution message appear. He then notifies me that the rudder was far left and the plane was flying uncoordinated. Only then did I realize I had inadvertently put in left rudder causing the fuel imbalance. I returned the airplane to coordinated flight and the fuel imbalance corrected and the caution message went away. We shot a visual approach and landed without further incident. The Flight Attendant let us know the smoke began to dissipate during our approach and finally cleared up during landing roll. The Captain and I determined that we didn't need to evacuate and continued taxi back to our gate with the fire trucks and police following. The Captain and I thought it may be de-icing fluid. But it was 50 degrees outside and the plane did not need de-icing. Maintenance told us the aircraft had been in maintenance over the weekend having work done on the lavatory. I don't believe I could've done anything to avoid the smoke coming into the cabin; but I do know that if I calmed myself down; I would've caught my inadvertent left rudder input before the Captain did. I'm glad he trapped it and let me know.