Hi, suffer from gallstones, been put on metronidazole 400mg for mouth infection, woke up today with severe back pain & feeling very nauseas and weak?

Not clear. Flagyl can make you very weak and sick if you take it with alcohol, but you should not get severe back pain from this. There are reports of pancreatitis and cholecystitis from flagyl, but these are very rare (and Flagyl is often used to treat acute cholecystitis). I would get in to see the physician who prescribed these to you as soon as possible, or go to the er if you can't get in today.
Nausea and back pain. Hi! it's possible the nausea is due to the metronidazole. Gallstones usually cause pain under the front right rib cage, but occasionally can radiate to the right flank in the back. You should see your primary care physician for an exam since you have a few different things going on. You may require blood tests to check for infection and liver/biliary function.