Worried and confused

Hello, I'm 18 and I had unprotected sex with my boyfriend on Wednesday, he didn't pull out or anything ( careless and silly I know) 
​My last period was on the 27th of September and it lasted about 6 days 
​My periods go through phases of being regular and irregular, but the Cycle is always between 27-31 days 
​I'm unsure about ovulation and fertility windows and I'm really
​Worried that but the end of this month I could be pregnant 
​Please
​Help