First, if you don't complete a prescribed course of antibiotics, the infection if you have one will come back and also be harder to control with antibiotics. I'm guessing your doctor didn't actually think it was an infection, or he wouldn't have put you on such a short course of antibiotics and certainly wouldn't have stopped it mid-course. It's also impossible to choose an appropriate antibiotic if you don't know what infection you have. He probably stopped it because he didn't think it was an infection and, if it was, that short a time probably wouldn't have been long enough for an antibiotic to work that well. Taking antibiotics has an adverse effect on the immune system, so using them isn't a great idea unless necessary, and if it was a sinus infection, they almost always go away on their own and don't need antibiotics. I have no idea what it is you do have but if you passed it to someone else, it really sounds like a virus. My first step would be to get another covid test, they are notoriously inaccurate and sometimes don't show up positive in the earliest stages of the infection. Other than that, if it isn't that, you need a diagnosis from someone, and maybe that has to be a better doc. I always find it odd when someone reports being given an antibiotic without a diagnosis. As to why your body hasn't fought it off, well, life is just like that. Bodies work until they don't. Peace.