I am 17 years old (male), and I suspect that I am a precursor of ankylosing spondylitis.

You tested the B-27, right? Negative should be nothing, not rigidity. Maybe I didn't have a good rest during that time, or I caught a cold. Just pay more attention to rest in the future, and sleep for more than 7 hours every day. Health is more important than anything else. If the body crosses, there is nothing. Take care!