Grapefruit juice contains a substance that increases the half-life of many drugs. It will therefore keep the levels of the antibiotic at higher levels for longer periods of time. Although antibiotics have some toxicity, taking grapefruit juice will not harm you at all and will increase the effectiveness of the antibiotic. Grapefruit juice should not be taken with other medications, specifically those that affect the heart rate or blood pressure.
I feel that you should call your doctor or even a pharmacist.
Best to call the local pharmacy.