Check engine light after smog check

You could not know for sure unless you have the codes check to find out why the light is on. Performing a smog test in and of itself cannot cause a light to occur. They do, however, as part of the test check your fuel cap, and if it is not securely fastened, the light could come on. I'm guessing you nay have gotten gas since the test, though, so that may be a non-issue. Start with code checks and go from there. Perhaps even ask the smog check station to check for you, since they recently passed your car, and it is odd to have a light come on now.