I'm not an expert or anything, but i'd think 20 days clean after only smoking a lil bud.....you should be fine. Especially if you know you were totally clean before the one day of indulgence. Exercise a bit, drink some extra water/gatorade and you should be fine.

Get an at home test.

When in doubt, sub it out!

My guess is that you should be fine mate. Just dont smoke again til after the test.