To put it in a nutshell, a child with a learning disability normally has an average to above average IQ (Intelligence Quotient), but struggles academically. This may also be displayed with behavioral or emotional problems. It is not at all unusual for a child with dyslexia symptoms (or other learning disability) to understand a concept one day, and completely forget it the next! This leads to extreme frustration on the part of the child (as well as the teacher or parent) and may result in behavioral meltdowns.
Please keep in mind though, that the behavior isn't the problem. The learning difficulty is.
That's why it is so important to get testing and intervention as early as possible.
Doing so will save you and your child a lot of grief.