You had sex on day 12 of your cycle. At that time you were likely very near your peak fertility.
Normally you will ovulate between days 11 and 16 of your cycle. Day 1 is when your period starts.
Sperm can survive for about 5 days.
When did you take the pregnancy test? Did you use your first morning urine, wake up pee, when you tested? It is VERY important for the best accuracy to do so. Also, use a name brand test, like EPT, not a cheap test.
I'd suggest you retest. By now it should be accurate.
Post back with the results and we'll go from there.