[jerf]

Imagine that something is dribbling fuel for a fire into a container. This is your intestines natural healing process. Then, every time you eat, someone tries to catch the fuel on fire. A untreated celiac patient's intestines don't "catch fire" with every meal with gluten in it, because there isn't always even enough fuel to light a fire. Waiting three weeks lets enough metaphorical fuel build up to catch on metaphorical fire. (This metaphor isn't perfect but should demonstrate the point.)

Three weeks is even a bit generous; two is frequently recommended.

If you can go without gluten for three weeks, then freely eat gluten again, it is almost certainly not celiac. Or it's very, very weak, and can't be wrecking your life like savemylife's.