Could be I suppose, but I would have guessed that it would take longer than a couple of days, more like a week or two, without Imuran for a flare up of disease to start due the to dropping Imuran levels.
Thus, I would suspect something else is coincidentally causing the diarrhea, However, I suggest you get back on the Imuran as soon as possible, since it is very likely a key drug in controlling your IBD.