Does Flucloxcillin Cause Stomach Upset?

I took 1000 mg of flucloxacillin every 6 hours for 7 days about 2 weeks ago. Suddenly I am experiencing a severe upset stomach which has been going on for a week now could this be caused by the tablets? I am a 64 year old female, 5 5 tall and weight 10stone 3lbs. I have previously had breast cancer and am currently taking femara letrozole.