Pregnancy symptoms

I just wondered how soon people knew they were pregnant before their missed period? I've been off BC for less than a month now, had unprotected sex a couple of times within my fertile window but according to glow today is my most fertile day but for the last couple ive been feeling off. Not unwell but not myself, I'm eating my body weight in whatever I can get my hands on and keep having dizzy spells... Any ideas? I don't honestly think I could be pregnant already!!!