When do I take a test?

I started my break week Monday and had sex Monday. I was told I was unprotected so was given plan b today (Wednesday). It's my 3rd day of my break week and I usually bleed on the 4th but I know plan b messes up periods. My nurse said for me to come back Friday and if I'm not bleeding to go to a clinic on Saturday to get a test. Would a test actually show anything this early??