Is this normal side effect during antibiotic treatment?

I’ve been silently lurking on this sub for a while. I was diagnosed with methane dominant sibo but was prescribed rifaxmin + metronidazole for a 2 week course (no neomycin because of pre-existing ear problems that an ent said would probably get worse on neomycin).

I’ve been taking these medicines 3x a day as prescribed. I had 0 side effects for the first 5 days. On days 6-8ish, I started getting moderately painful heartburn that went away with tums. My problem began, though, on day 11ish, where I woke up around 1-2 hours after my dose, with a small amount of vomiting, shaking/chills, dizzy, and a severe pain in my chest and the feeling like I couldn’t breathe. It was so extreme, I went to the ER. They did some tests and a chest xray, said everything looked fine, and only found that I was dehydrated and gave me fluids. The pain went down around 2 hours after and I was given some Zofran. I was able to reach my GI who told me to take the zofran and “try to hold on”.

Then, the same thing happened after my next dose. Same extreme pain and all minus the vomiting. Started around 1 hour after dosage and lasted around 2 hours. I took the next dose, and the same thing happened again. After hours Dr. at my GI’s hospital told me to skip the next dose. I did and felt fine. The next day, I took another dose and same thing happened again.

Clearly, I’ve suddenly got a reaction to the medicine. Is this common? Has anyone been on these two medicines and together and felt the same? I technically have 5 days left of the treatment but the feeling/pain is indescribable and I’m not sure if I can continue to “hold on”. Is this common and I should just really try to power through?

TLDR: I was diagnosed with methane-dominant SIBO from t test, only SIBO symptom is pain after eating. I was prescribed xifaxan and metronidazole and was fine at first, then started experiencing heartburn, then began experiencing insane chest pain after each dose. Is this normal on these pills or should I absolutely stop?