Does Doxycycline For Lyme Disease Cause Rashes On Neck And Mood Swings?

Hi, I am a 35-year-old male. I have experienced achy knees, especially in the morning, aching in lower legs and serious fatigue for the past 18 months. These symptoms first began after I was bitten by a tick in an area that in which Lyme disease is endemic. My doctor prescribed doxycycline one month ago as a precaution and tested me for Lyme disease. After I started taking this I experienced new symptoms: a rash in a ring around my neck and red face, as well as severe mood swings and very occasionally an achy feeling in my ankle, lower arms and hands. My original symptoms and the new ones seem to be reducing and especially the fatigue is much reduced. However my test results for Lyme disease have come back negative. My reaction to doxycycline makes me think that I must have an infection that it is acting on, but as my test results were negative Lyme disease has been ruled out. I ve had all the other standard health checks, which were fine. Is there any way to explain my reaction to doxycycline if I do not have Lyme disease? Also I have heard that sometimes people can test negative even if they have it. My GP has ruled out Lyme disease but I really feel like it is the only thing that explains my situation and if I have it I want to make sure I have the proper treatment. I would really like to here an opinion from a healthcare professional who has experience of Lyme disease as I do not know what to do.