Pls help! had unprotected sex on 1st jan but period came that night and lasted approx 4days. today realized a little spotting. pregnancy or old blood?

Possibly: Pregnancy is always a possibility. Home pregnancy tests become positive at about the time you'd be one week late. Perform one then and repeat in a week it it's negative.

Do pregnancy test: The easiest way to find out is to do a urine pregnancy test now as it will be positive if you got pregnant on jan 1. Be sure to avoid unprotected sex in the future!