Talking With People

Hi, I'm just going to rant about my job rlly quick. While at work today, I come in as usual. My manager says hi to me, I say hi back. He asks me if something is wrong? I say no, I'm fine. The truth is that I actually have a cold, but I didn't want anyone to know because I didn't want to get sent home. My manager didnt seem to belive me when i said im fine, but he said "okay." He pulled me aside towards the end of my shift and asked me what's wrong again. He asked what's going on with me because I was in a bad mood earlier and he asked if there's anything he could do to help? Looking back on it he was just trying to be nice, but I got a little defensive and kept telling him nothing was wrong. He often asks me of I'm okay, or if something is wrong though. It wasn't just today, but today he was very adamant about it. I am rather quiet and serious while working so maybe he thinks I'm in a bad mood because I'm quiet? I'm not sure, but I really don't want to leave a bad impression on my coworkers and seem unfriendly to them. This is why im worried about this situation! so I just wanted to rant about it! I will try to smile and speak more energetically while at work so hopefully my manager will stop aking me if something is wrong and worrying about me!