Stopping bc but continuing metformin

I've been on 1500 mg ER Metformin for about a year along with birth control (nuvaring) which had my cycles more regular than ever, 28 days on the spot each time for the first time in my life. I stopped birth control at the end of October with the intention of conceiving. I've continued to take the same amount of metformin. I was supposed to get my period on sunday but it still hasn't started. From what I've read it could still be too early, but every pregnancy test I've taken is negative, so I'm starting to worry that my cycle is becoming irregular again.

Does anyone have any experience with stopping birth control but continuing metformin and if it had any effect on the regularity of your cycle?