confused!!

So I don't really understand the whole due date thing, my last period was Jan 5 but I have a 36-40 day cycle. I know the exact date I got pregnant which was Jan 21st. Putting in the date of my period in both the app and my doctor says I would be about 6 weeks, even if I change it to a week later due to my longer cycle it would be 5 weeks yet I got pregnant exactly 3 weeks and 4 days ago. Also the app is sayingat  3.5 weeks I wouldn't get a positive test result, yet I got a very strong positive at exactly two weeks. I've read somewhere that you count starting a week after the first day of your last period, so how far along am I really??