not sure what's wrong

Hi , I am a freshman in college and have just noticed a problem within the last few months. At first, I thought that it was just a temporary problem and that it would go away eventually. However, it has been on my mind a lot lately and shows no signs of going away. For some reason, I have been having difficulties speaking. There are times when I can't think of what to say or how to say it. This not only applies to grammatical structures but also to idiomatic expressions and phrases. I have always been a native English speaker, and I don't know why I am dealing with this now. It is really scaring me because I feel as if I am expected to be at the top of my game, both in school and in my personal life. I don't know if this is just an aspect of my perfectionist personality or if I am really dealing with a learning disability. Thanks for any assistance.