I took doxycycline twice a day about 2 1/2 weeks ago for 7 days for an STD it turns out I didn't have(mixed up test results). I had horrible nausea and headaches every day. I had no appetite until late at night. My legs were really itchy too. Now my period is late for the first time in 3 years!! I know I'm not pregnant because I havn't had sex since my last period and I think it has to be because of this medicine.