Heartburn?

Hi guys, so I had sex with SO during my fertile window. We aren't trying and was an oopsie. I took plan b about 23 hrs later, but I am currently experiencing horrible heartburn and feel wiped out. I think it's only around 3dpo for me, but could this be a sign of pregnancy? I know plan b is not 100% effective, but I'm just curious what other people experienced around this time.