Erythromycin is an antibiotic, not Prep or PEP. It's extremely well known to cause really bad stomach pain, so no wonder your stomach is hurting. I won't even take erythromycin anymore because of the stomach pain side effect. And no wonder you can't eat because that pill will make you nauseous. Why did you take pills that someone who is NOT a doctor gave you? That wasn't smart. Unless prescribed by a doctor specific to you, don't take random pills that someone gives you because you never know what they are giving you or whether you could be allergic to it or something!
If she always uses a condom then she would be negative. Try to relax until it is time for you to test. A duo test is conclusive after 4 weeks, and any other test after 12 weeks is conclusive.
Billions of people get a sore throat but not many of them have HIV so you need to stop examining your body because symptoms can't prove anything.