Losing weight quickly in one week? (not the first week)

Before you assume this is water weight from starting a new diet, here's my story. I started back in July and the first week I lost 6 pounds. The next was 3 and then it slowed down between .5-1.0 pound. I've lost a total of 22 pounds so far in the past 2 months and feeling great. I eat at least over 1200 net calories (my overall calories range from 1400 on a rest day to 2000 on a workout day) with an average of 1600-1800 calories. Also I lost inches all over my body, with my chest seeing the most loss. As a matter of fact, I've dropped a dress size.

Then this week happened and I was 220 pounds. I thought, yeah it's going to be another week I'll lose a pound. No big deal. However, I weigh myself yesterday and is down to 218 pounds. I wasn't feeling to well but still managed to eat and exercise. Then this morning, it was 216. I'm still not feeling well and decided to have a "rest day" today but the weight kept on dropping. Is it from the weeks of losing .5 pounds catching up? Is it because I'm sick? Is it because of getting PMS next week? I really don't get it. I felt I might've lost some lean muscle as well as fat but I don't want to lose so quickly in a week, especially if I'm eating as much as I can to lose no more than 2 pounds a week.