I'm in high school, and throught my life I've done well academically. In the past year, I've been getting worse grades and it's time for me to apply to University. I know I need to do better, I'm just not motivated. Also for a couple of years have not enjoyed hanging out with my friends and I honestly try to avoid it by usually lying that I just am busy with family stuff. 

Recently I went to the doctor for a physical and they asked me about how I was doing at school and stuff and I told them about my grades. They seemed kind of concerned about it, and put the idea that I might want to go talk to someone about it, because there might be an underlying issue.

My stepmom also thinks that something could possibly be wrong, but I don't think she wants to admit it to herself because of how she has seen mental illness in her family. 

Basically, I don't know what to do. Do I have a some sort of mental illness? I'm seeing my doctor again in a month for a follow up of a prescription I got (skin cream, nothing related), and should I ask them about it more. I told them that everything was fine, but now thinking about it more I don't know if it is.