Hi, Lala,
Most antibiotics do not decrease the effectiveness of pills, with exception for rifampin. Studies clearly show that rifampin decreases the effectiveness of birth control pills in preventing ovulation. Other antibiotics, particularly
penicillins (like amoxicillin) and tetracyclines, could impair the effectiveness of birth control pills. So, Amoxicillin can have some impairing effect, but unlikely significantly reduce the effectiveness of the pills.
Likelyhood of pregnancy in your situation will be low, slightly higher than if you wouldn't take antibiotic.