Welcome to HCM.
You had tests for STD
and they came negative. But you had not mentioned what type of test you had -Whether an antibody test or Antigen test or DNA PCR and at what duration after your risky contact it was done. If it is antibody test you have to do the test after 3 months to rule out completely. Antigen tests and PCR test will be sensitive as early as three weeks. If your tests are reliably negative then you need not worry and all your symptoms need not to be due to HIV. Flu
or throat infection
may be the reason and loose stools may be the side effect of antibiotics. If you have scrotal pain consult your physician and have the investigations done and get it cured. Stick to safe sex
always so that you will not have unnecessary tension.