Looking at all the dates you had sex, there's no one that could tell you who is the father hun, it would depend on when you ovulated. Given the spot bleeding, which may have been implantation bleeding, you could be far enough along in the pregnancy from having sex with the guy that was not your boyfriend, but like I said, I have really no idea, I'm just guessing here.
You will have to get a paternity test when the baby is born. They are very easy and simple to get. All the best to you and your baby.