I'd say like .0001% chance.. when your 16 your cycle acts all fucked up, cause Its just starting and beginning to become consistant, explaining period one week early. The old blood is also common sometimes in girls, she acted like it was nothing because shes probably used to it. Implantation bleeding isn't normally brown.. its fresh blood. Also, don't use this as a form of birth control by any means, but typically theres only a couple days a month where a girl can even get pregnant, there different for each girl but usually fall about halfway through cycle, neither directly before or immidiately after her period, but this isn't fool proof just extremely unlikely. I'm not psychic though, just interpreting what your saying. The only way to know 100% is to take a test. They sell them at your local walmart, and its not like you have to show ID to get one.
On a different note, you mentioned it was 100% protected.. until your ready to discuss your concerns and questions with your partner, and are ready to understand that any time you have sex you take those risks, and aren't losing sleep over the possibility after "100% protected sex" your not readdy to have sex.
