Should I use an additional method of contraception with clindamycin?

Hello, I take vibin birth control pills. From 9 to 12 tablets I took the antibiotic clindacymin, which reduces the effectiveness of tablets. With additional protection throughout the cycle (before, during and after the antibiotic) and bleeding from withdrawal on time (4 days placebo) can I assume that the tablets are working properly? Is there no reason to fear pregnancy in case of withdrawal bleeding?