Thank you for your query.
If you had sex for the first time , the bleeding could be because of rupture of the hymen
which is common in virgins.
Hence, please do not freak out about it.
Also, sometimes cervical erosions, polyps or growths could cause such heavy bleeding.
, you would know only if you miss your period.
Immediately the day after intercourse, it is IMPOSSIBLE to say whether you are pregnant.
Please go for protected intercourse
in the future, to protect against pregnancy and STDs.