Pregnancy or Late AF??

I have a question it may sound crazy but how did you know when you were pregnant? I was on nexplanon for 2 maybe 3 months at the most and I've been off it now for a month and my doctor prescribed me prenatals that I've been taking starting Oct 12 when nexplanon came out and my AF was regular on nexplanon just lasted longer than 4 days like Normal more like 2.5 weeks... AF expected on the 8th and now I'm about to be 4 days late and I have mild cramps and tender breast a lil nauseous at random times of the day or with certain foods should I buy a HPT or wait a little longer?