If your condom didn't break or slip and you used it during the whole intercourse, then you used it correctly. If I were you, I wouldn't have even gotten tested, and I'd trust your result at 78 days.
Hello, if you used a condom properly you didn't even needed to get tested in the first place, they call it safe sex for a reason. You can trust your tests and 80 days would be too long for ARS symptoms.
You have nothing to worry about. Some times a cold is just a cold.
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