What Causes Acid Reflux With Black Stools After Using Doxycycline?

every since ive taken doxcycline i think ive developed some sort of reaction to it. ive researched my symptoms online and they all say acid reflux or possibly and ulcer but point toward acid reflux more. i stopped taking it about a week ago and im still miserable especially if i eat after 5pm. my stools are black but ive been taking pepto bismol everyday else i get to the point where im fighting nausea and potential vomiting. its like the only thing thats helping me but tonight i still felet pretty bad. i dont have insurance and i dont know whats wrong with me. what should i do?