You might just be so stressed that you're attributing this to a possible pregnancy. When was your last period? Most women are the most fertile at the half way(14 and up) point of the 28 days cycle. If you were having relations at this point, then there's a slight chance. Most women also get breast tenderness at the half way point. Some women who are very in tune to their bodies can tell early on when they're pregnant. If you're not, take this as a learning lesson and never rely on the pull out method. You should be using a condom to avoid STD's but also an effective birth control method. I'll be keeping my fingers crossed for you.