Around days 7,14, and 21 tend to be heavier detoxing days which may explain the symptoms around that time.
The way I know when I am done with a cleanse is I get the feeling of being tired of the whole thing.
This is different from going through detox symptoms. I can feel rough as heck but I'm not tired of it. When that is happening I want to keep going.
I usually find this point of being tired of it right after a day or two of feeling quite wonderful.
After the heavy detox days 7-9, were there any days of feeling quite good?
I going to guess that you may have gotten a lot done already on this cleanse and may possibly be done with it.