I came on my period on thursday evening - i went out on the saturday and drank a lot of alcohol i then had sex and my period has disappeared why?

Coincidence: You just had a short period this month. Alcohol and intercourse would not affect your cycle so this was coincidental. Please be careful overindulging in alcohol especially when combining it with sex. I have seen so many people do things they regret as well as put themself in seriously risky situations. Take care of yourself.

Get a pregnancy test: A classic reason for a short menses is pregnancy, but there is often variation between cycles. Drinking a lot of alcohol is never a good idea, especially for women whose livers are more susceptible to liver damage. The liver effect of alcohol could have displaced estrogen metabolism and thereby shortened your menses.