I have been sick with side effects from Doxycycline for two weeks. I had a UTI which has cleared. I have been to the doctor twice, they checked my urine, and there is no infection. I feel tired, have chills, I feel cold, have a back and joint pain. I do not have a fever. I have read that, sometimes it can last up to three weeks. Is it possible to last this long? My blood tests are also normal.