Metformin

Lindsey • I`m engaged to the love of my life & we are hoping to have a mini us sometime in the very near future.
So, January 11th I got AF & it ended on January 14th, on January 15th, I started taking 500mg of Metformin a day. Yesterday, January 28th, was the 18th day of my cycle & I started bleeding, very lightly, like the beginning of a period. I noticed it around 4:15pm & is still going.
What could this be? Any ideas? Please?