What Causes Irregular Periods?

i had my period twice this month one around the first of the month and another towards the end. i think it has something to do with pregnancy but last time i had unprotected sex was 4 days ago and it wasnt even that long, maybe for 1 minute because i was scared of getting pregnant, but also i heard this could be from stress too. i have been stressed out alot this month so i need help to determine which one is more possible