Question: Metformin and Implantation?

This has been driving me crazy!!! Me and my husband have been ttc for bout 6 months. I went to see my doctor and she put me on Metformin 500 mg a day. I am not diabetic but she said it will regulate my periods and help me get pregnant. I started taking Metformin in Feb.2016 and continued on taking the pills. My period was back to normal! YAY and I was super excited. I kept track of my periods on the My Days app and had intercourse on HIGH FERTILE Days and Ovulation Days. Well I was expecting a period on the 4th of May and it came but it wasn't normal. I was spotting on and off and it lasted for about 4 days and there wouldn't be any blood on my panti liner. I took a pregnancy Test and it was a BFN. so I'am confused. I read online that it could be implantation bleeding but I would like to know how long would it take to get a positive pregnancy test after implantation bleeding?