This is very common in dyslexic people, especially through school. I had real trouble with the knowledge that i understood everything i was supposed to and often much more than was required for my age group, but could never explain it.
This often lead to teachers patronisingly simplifying the material down and treating me like i was completely stupid, as though i had nothing but hot air between my ears.
This combined with under achieving in exams make it very easy for dyslexic students to simply say.... right maybe i am just stupid!
This as a rule couldn't be further from the truth.
Although at times dyslexia seems to be a huge curse. There are a ridiculous number of advantages to being dyslexic.For example dyslexic people as a whole are extremely creative and intelligent people. They have the ability to think outside the box and approach problems differently. I believe there is a book called the 'gift of dyslexia' which could help you understand the benefits further. I mean some of the most intelligent people in history, such as Albert Einstein and Leonardo De Vinvi, both showed many signs of being dyslexic.
Anyways i hope this helps.