LMP due date and PLAN B???

So I took a plan b in August (we chickened out lol) and had an early period in sept. Usually AF is 20th like clockwork.. but sept it was the 15th, I guess bc plan b? Well now I'm pregnant, and according to my LMP due date I'm 6 weeks.. but my blood work was showing 3-4 last week, and now 4-5 this week. Will this change my due date?? I'm so confused on how far I really am. All Of my pregnancy apps are apparently off now, and it's frustrating not being able to keep up!