I never was asked to show my previous 90 days confirmation when doing my report in person. They have all the details in their system.
You will get an email reminder approximately 1 month before your next report is due. This way you'll know the date, unless you went abroad in the last 2 months.
"returned before the 90 day report was due"? Why? The 90 days are reset when you re-entered the country. Did they never tell you at immigration you were way too early?