15dpo, period late for 2d. tried test 3 times during 6 days all negative. what are the chance of pregnancy? can a test become positive later on?

Depends on test date: Symptoms such as nausea (with or without vomiting), some fatigue (feeling less energetic), a sharper/stronger sense of smell, mild soreness/swelling of the breasts, and maybe some food aversions and cravings, can happen with other conditions, not just with pregnancy. However, if the pregnancy test was done less than 2 weeks after possible conception, a woman can repeat the test a week later.