Maybe data will help you to relax, djr5a. When an egg is fertilized it begins a journey toward the uterus and will implant itself in the uterus triggering the body to begin formation of B-hCG. Pregnancy tests (blood and urine) are based on detecting B-hCG in levels that indicate pregnancy. Blood tests are more sensitive to early pregnancy than urine tests because it takes a higher level of B-hCG to spill over into the urine. Blood tests can detect minute amounts of B-hCG and any amount higher than 5mIU/mL is indicative of an early pregnancy. Urine tests have a sensitivity of between 15 and 25 mIU/mL. Once the egg is implanted B-hCG production skyrockets. Here is a
Table showing detected levels of B-hCG in pregnant women by five different labs. The table is based on number of weeks since conception. You'll notice that by the end of the second week the levels of B-hCG are sufficient to be detected by urine tests.
Let's imagine the worst possible scenario. That would be that her bcps failed and your condom failed and that there were viable sperm available to meet her egg if she ovulated. Her ovulation would have to have happened -1 to 5 days from the act in order to meet viable sperm and still be fertile. Eggs are typically only fertile for about 24 hours after being released from the ovary. For our example let's use the longest lead time (the time that would allow for the least amount of B-hCG production at the time of her pregnancy test at the ER).
Let's say you had sex on Saturday and the condom failed. Let's also assume that she ovulated 5 days later, even though she was on birth control, and your virulent sperm were still swimming around looking for an egg. Then this fertilized egg started slowly making it's way towards the uterus and implanted 8 days later (on day 13). She would have immediately begun production of B-hCG and would have produced enough by the end of the next week (day 20) to turn every pregnancy test on the planet positive. This is why pregnancy tests are accurate from as early as one day beyond a missed period. So, if the negative pregnancy test done in the ER was 20 or more days after the last time you had sex then she is not pregnant with your child.