I am assuming that you are using the My Cycles app.
On the bottom of the calendar (accessible by tapping the three lines in the upper left corner and then tapping My Calendar) there is a key for the different colors.
The red color is for when you are on your period (menstrual cycle). The green days are indicative of when you are likely to be fertile (leading up to and the day of expected ovulation). Those days are best if you are trying to conceive. If there is a red and white striped line on your calendar, that is when the app predicts that your next period should start.
There are a number of reasons why your cycle could be off. Stress, illness, hormone problems, etc. can all make you late. If you had unprotected sex during your fertile time, there is a chance you are pregnant and should take a test. That is really the only way to know for sure.
Best wishes to you and if you have any other questions, please don't hesitate to ask.
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