What Causes Missed Period While UPT Is Negative?

Hello, I've missed my period by almost one month. I took a pregnancy test two weeks ago and it came back negative. But that felt like forever ago. I don't "feel" pregnant and I could be stressed because I'm in China at the moment and I'm planning to move back home and coordinate a job. Also, last month I had pneumonia due to the terrible pollution here and I had to take some drugs and I'm wondering if that has affected me at all. Help!