My teacher thinks I am depressed, how do I convince him otherwise?

I have just been tired out and exhausted this entire week. A couple of days ago I went to see him about my work and he said I looked like I had been crying and asked me if anything was wrong. I told him I had just bombed a physics exam was all, but it sounded like a lie, and it was sort of… so he asked me again if I was depressed about anything and I told him no.

That night I fainted and cut my lip open, came into college the next day with a busted lip and the first time I saw him that day was when I was running to the infirmary in tears, he said ‘hello’, saw I was upset and awkwardly asked me if I was coming to the extra class at lunch while I was buzzing into the infirmary and I snapped at him that of course I was.

I don’t want him to worry about me as he is the only teacher I get on with and just have fun conversations with. It is nearing exam leave and I don’t want the remaining time in college to just be consisting of concerned looks from and awkward conversations with him.

I am kind of down at the moment but so is everyone my age all over the country going through the same thing. I just want to show him that I am happy and get back to our jokey, light exchanges. But I don’t know how. It would be odd if I smiled all the time and seem like I am trying too hard, but I don’t know how to not think about it and make it seem natural. I have already decided to dress sharper and be made up like I usually was before the last week rather than bare faced with scraggy t-shirts, but that’s all I can think of.

Any suggestions for ways for me to feel content enough to come across as okay? Odd question. Sorry. But I need help.