Pregnant?!

So almost a week ago I had sex with my fwb. We used a condom but it kept coming off it was either too big or he was not hard enough. He finished inside of me. I was on my first fertile day which I didn't know until later that day. I know the app is not always right. I'm due for my period in about 7 days. Is there anyway I could be pregnant. I won't be really worried until I miss my period. Also when is the best time to take a test?