Taking a pregnancy test the day after you have sex isn't going to work. The egg may not be fertilized even up to a few days after, depending on when you ovulate. The pregnancy hormones build up in your system, but they are only detectable by a test after you miss a period, simply because they aren't strong enough yet.
You have the take the test after you miss your period. Taking one after a period is pointless: having a period is like an automatic negative pregnancy test.
Since you had a period after you had sex with the "other boy" and didn't have sex with him after that, then you are not pregnant from that relationship. It doesn't matter if the dates are close. Don't feel stupid either. Sometimes when you are pregnant and stressed out, it fogs your mind. It's completely understandable.
Your baby is your current partners. I wish you all the best with your pregnancy.