My period hasn’t come yet ?

So I’m taking this pill called metformin which is used to treat pre diabetics like me. So I’m mid December I bleed for two days and my normal period began 3-7th of December. Now that’s it’s January and I began taking this pill December 26th im late ??? Mind you I’m sexually active and use condoms and check them every time they are used whether we water check them or we blow air into it to check if there is any air bubbles. Who else takes metformin ?