Bacteria Sinusitis resistiant to Amoxicillin

For over 3 weeks now I had post nasal drainage. I went to my doctor and he ran a CBC and said everything was normal so at that time I had no infection. I questioned him about a sexual protected encounter I had and the possibility of it being HIV but he said since it had been a year, something would be a little abnormal i the CBC. Then it turned into nasal congestion and progressively is getting worse. I had been coughing up lots of green mucus and such and had a sore throat for 1 week. The my doctor called me in some Amoxicillin 875/Culvanate Potassium 125 and I felt a bit better for 2 days and now it's the 6th day and its bad again. I have sever nasal congestion blowing out clear stuff and still coughing up yellow stuff and my throat has been severely hurting for the past 2 days. I have taken Amoxicillin 3 months before for another sinus infection/bronchitis which I get a lot. But I missed a lot of doses and such. Then I got Strep a month later and he gave me the same antibiotic and it cleared it up, but it took over 4 days.

Could it be possible that I have a resistiance to this Amoxicillin? I am going back to my doctor on Monday because I also have been having sporadic low grade temperatures of 99/100.5. What is another Antibiotic that I could ask him about that may be stronger and me not be resistant to?

Thanks so much everyone for your help!