I suggest you not to worry about your periods. Delay in periods is quite common in people who use Ipill contraceptive. Sometimes this delay may long up to fifteen days than your normal periodic cycle. Using Ipill causes hormone instability that may temporarily change your periodic cycle.
As in your case,your started on 21st, so 31st is very near to the window fertile period where chances of getting pregnant are higher. I also advise you to take the contraceptive pill within 24 hours. Even though the pill may be taken in between 48 hours after the act, it is strongly recommended to use Ipill before 24 hours to decrease the chances of getting pregnant.
As you have 3 pregnancy tests that eventually turned negative, the contraceptive might have worked well. Do repeat the pregnancy test after a week with the early morning sample. I advice you to avoid stress and relax well. Ditch that anxiety in waiting for your period everyday. Just stay strong and be confident. Avoiding stress makes it easy to normalize the harmonic surge and replenishes the normal periodic cycles.
Don't worry your periods are coming soon. I advice you to use alternative contraceptive such as condoms instead of Ipill. That's healthy for you. Use Ipill only in case you suspect failure of condoms.
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