Hi Minervah,
Lucky for you, there are a few things going on.
1. Summertime is a time of stomach viruses. We eat more fruits (with bacteria in them), drink more water (which dilutes stomahc acid) and are generally more prone to stomach issues.
2. Your period may not have totally established itself and you may not be ovulating after coming off birth control.
3. It sounds like you're doing some "family planning". Just remember that the most fertile days are about 14 days BEFORE your next period is due to arrive. In your case, this is probably somewhere between Days 20-25.
I'd suggest that you wait for a little while longer, and continue to test at home. If in a week, you still don't have your period, consult with your GYN for a blood test or ultrasound to confirm or exclude pregnancy, and to also report the delayed period. Good luck! Please update.