I remember years ago I was at my heaviest, that would be probably about 205 pounds, it shows in photos, my face was really chubby. I didn't even drink at the time. Not that I do now, but it was later that I developed a taste for alcohol.
So my friends told me I had to drink more water and eat a little better, and as soon as I started taking it seriously I was losing weight at about that rate.
So it could just be water weight you're losing fast, but don't take my word for it.
I ought to be riding my bike every day but I'm the laziest person on earth.
For the record I'm 5' 6", currently 183 pounds.