Did you confirm that high reading by washing your hands and repeating the test? 20 seems exceptionally high 2hrs after a meal. It's possible to have a out of date or faulty test strip or maybe something on your hands causing a high result.
What was the meal that caused such a high reading, low carb, high fibre doesn't give us much idea of what could have caused your spike. Fibre is just another form of carbohydrate and although it does not get digested and just passes through, some high fibre foods also contain lots of carbs that do get digested, oats, sweat potatoes, beans, quinoa, wholemeal bread, to name just a few.