The stomach issues are probably the antibiotic, and if they get worse (diarrhea, fever, pain) you should make sure you don't have C. difficile infection; it's getting increasingly common-- at my hospital, 1/100 people on antibiotics end up with it. But usually it's just regular old antibiotic associated diarrhea, which gets better when the drug stops. The tendon rupture risk is only with fluroquonolones (ciprofloxacin, levofloxacin), and is more pronounced in older patients and those on prednisone-like drugs, and even then the risk is quite low. I'm guessing you're on trimethoprim/sulfa, which has no risk of tendon issues, and is pretty low on the C. diff risk too.