Grades are the gift you give yourself. Don't confuse grades with knowledge. A low grade doesn't mean you don't know something and a high one doesn't mean you do. High grades don't mean you are smart. Low grades don't mean you are dumb.
Many factors contribute to a grade that have nothing to do with intelligence.
Don't be surprised if you can't make fantastic grades in all of your classes. If you think about it, very few people are skilled in all of their subject areas. It is flawed thinking to expect that you'll be great in everything. Though we don't like it at the time, It is good to be challenged in areas that we have little skill or ability. One of the greatest urban legends of all time is about some student who makes fantastic grades with little or no effort.