how strong were the hints in the email? any chance of a false positive? thw way i see it, the only 2 people in your office who know are you and her...
if i were you, i would approach the girl who jumped into your bed immediately, befriend her, and with sincerity say "i know that youre not broken up with your bf properly, and havnt let anyone know about the night we spent" then ask if she has confided in anyone. then go from there.
imo relax anyway. the hot girl in the office throwing herself at you raises your value anyway...even to your ex...although that one is a bit fragile. itll just take a bit more talking to soothe her anyway
and if you think thats intrusive, i had people from my city (that i went on vacation with) calling my girl in ANOTHER city (2000miles away), asking if she had done anything with me. it was beyond childlike. i mean sure, guage if someone is still available, but to call them from across teh continent to ask if she had had sex with someone? holy fuck social illiteracy at its finest