Had sex a few days before period, period is now days late, three pregnancy tests read negative. What could this mean?

Just late period... If u had sex just before period. - you were unlikely ovulating.. you were past ovulation and so you could not get pregnant at that time. Consider Birth control and always use condom for STD prevention ... if not stable relationship. If any doubt check pregnancy test again and/ or talk to your doc. Most likely your period is just late. It can happen time to time. .
Missed period. It is not uncommon to miss a period. If three pregnancy test have read negative, you don't need to retest. Keep a calendar of your periods (there are several free apps for this) and if you miss your next period see your Dr. Stress, increased exercise with at loss are just a few causes of skipped periods. Be sure to use birth control unless you're trying to conceive and use condoms to prevent STDs.