Could I be pregnant?

Ok guys, so this is going to be a pretty detailed question so bare with me.... So I had unprotected sex on the 27th of December

. My last periods start and end dates were DEC. 20th (start)-DEC. 26th (end). So I had unprotected sex the day after my last period. My cycle is usually every 28-29 days, so I should’ve started around the 16-17th of this month, I STARTED SPOTTING INSTEAD! The spotting is dark brown in color, and here I am 2-3 days into my (supposed) period and I’m only spotting. Is this a sign of pregnancy? And when should I grab a preg test?