concerned if my girlfriend is pregnant or not

My Girlfriends Period Has Lasted 15 Days and Concerned about the odds if it is due to pregnancy. We Had Sex on the 17th of April and Her Period Began on the 19th she was a week late, she hasn’t received any of, if at all any of early symptoms people can experience at Week 1 and 2. Fairly certain the most that happened during Sex was Pre-Ejaculation, She told me she Ovulated on the 1st or 6th of April. Yesterday Her Blood Became Lighter and Browner which coincidentally, happened in the time frame implantation bleeding usually occurs. She hasn’t been able to take a test yet though and most symptoms that most people receive during pregnancy also fall in line with her usual period symptoms. I’m sorry it probably seems like an easy and open question but just the chances or possibility scares us both. Can anyone tell me odds or give me more info that can help determine whether she is pregnant or not. She has also been on birth control for a little while and she is fairly consistent.