What is the chance of pregnancy if i had protected sex with a condom (no apparent leakage/tear) 6 hours before my last period started? i am 3 days away from my next period and im experiencing heartburn and upset stomach. my average cycle is 28 days?

From the information you provided the risk of pregnancy is remote. If you miss your period by 4-5 days, do a home pregnancy test. If the result is negative repeat the test 4-5 days later unless you get a period by then. Use first morning urine and follow the instructions carefully. Get HPV vaccine. Safe sex: https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines