You may have been 'contaminated'; which basically means you accidently ate gluten. Staying gluten free 100% is extremely hard. You have to be careful, because 'contamination' can occur at any time in any food (even some food you do not expect).

For me, it took a few months GF before being able to eat a little bit of gluten here and there and not being too affected by it. However that is not an excuse to go out to Olive Garden (lol!!), it just gives me a little wiggle room in case of contamination of gluten.

Also: depending on how sensitive you are, you might have to eliminate other foods (like caffeine, processed food or other oats), and you may have to be even more careful to eliminate gluten out of your diet completely. It's difficult, but it is possible. And very worth it

Good luck
Sweets

PS remember that "Gluten Free" labeled food does not necessarily mean completely gluten free. It just means it either it is only '20 parts per million' or it is 'not made with gluten'. That basically means there might be gluten in it; it might be made in the same facility as gluten products or it might contain gluten during the process.