Hi
Welcome to the forum!
On Seasonique, ideally the periods should come every 12 weeks. As reported by you, you took 500mg of penicillin 4 times a day for 5 days, had unprotected sex and bled from vagina 2 days after the sex. The bleeding is bright red.
There are various possibilities. One is that the antibiotic caused a fluctuation in the hormonal level resulting in withdrawal bleeding. Normally Seasonique prevents ovulation. However it is possible that with fluctuation in hormone level ovulation occurred. Chances are slim.
Other causes for vaginal bleeding after sex are STDs like Chlamydia, gonorrhea, and trichomoniasis, cervical polyps, yeast infections, endometriosis, cervical dysplasia which is a precancerous condition, uterine polyps, uterine fibroids and cervicitis. None of them are conditions that should be ignored. Also with a history of unprotected sex STDs should be ruled out.
Discuss these possibilities with your gynecologist. It is difficult to comment beyond this on net without examining. Take care!