What Causes Light Vaginal Bleeding After Having Sexual Intercourse?

Hi doctor! sorry to bother but i may need some advice. i had sex last night, which is my 1st time and without protection. So what i need to do now? Actually i m not sure about whether there is something happened, but i found a little blood when i went to the washing room after it. About that, what should i do now? my friends suggest me to do some body check and find some bill named 'Post day'. i don't know what's that for and have no idea whether or not i should be that worried.

Pls share some opinions. thx