Had sex on day 6 of cycle. Period not due for another 3 weeks but I have pain in breasts/armpits. How likely is it that I am pregnant? Too soon for test

Not likely on Day 6. A urine pregnancy test done correctly will give a reliable result 3 weeks after sex (or when conception might have happened) or 1 week after missing a period. If done sooner, a positive result is likely real, but a negative result may be due to testing too soon, and therefore lead to a repeat test. If a woman has her normal regular period, then she is not pregnant, and the test is not needed.