I just took an antibiotic for ureaplasma called doxycycline mono. after about 5-10 mins, i burped and it seemed like smoke (or gas? or stomach gas) came out. i am not sure if this has ever happened before, is it normal? hasn't happened again since?

Ureaplasma is normal in the genital tract (half of all men your age test positive at any point in time). If you have nongonococcal urethritis (NGU), i.e. symptoms like urethral discharge, doxycycline is the correct treatment, but it's unnecessary just because of a positive test for ureaplasma. Anyway, doxy can cause stomach upset; maybe you burped up doxy powder. No harm or worry. Take with food.