I'm on yasmin (drospirenone and ethinyl estradiol) continuously. I had unprotected sex on the 4th day of my placebo pills, took Plan B 2 days later. Started my period but it lasted 1 day?

Okay. There's a very high likelihood that you're okay. 1) you had sex on your placebo pills, so low likelihood of pregnancy there. 2) you took plan b, so you double covered yourself as well. It's okay that your period only lasted 1 day, sometimes that can happen when women take plan b. You can take a pregnancy test in 15 days or so, but it will likely be negative, good luck.