possible positive?

So I use this app and according to it, I began ovulation on a certain day. If it's accurate, then my husband and I had sex my first two days of ovulating. It's two weeks later and I've had really sore boobies. It's either pms because I'm due to start on the 15th, or I've got a bun. So I took a pregnancy test and got a really faint second line two days ago. Since, I've taken three more and they've been negative. I know it's early and you can rarely test positive but I've heard that there's no such thing as a false positive. Thoughts or advice?  Thanks so much!