I need help figuring out due date

Okay so I am definitely pregnant. This will be my 3rd baby! I have almost 5 years old and almost 2 years old boys. My husband and father I weren't trying to get pregnant but also didn't think I could because I have crazy cycles. I honestly don't remember the last time I had a period. I think it was April sometime but it was super unusually (for me) light and short. I've got a long history with messed up cycles so I just stopped giving a damn about keep track of anything. Anywho, we had unprotected sex one time assuming that we couldn't get pregnant or probably wouldn't. And here we are. I didn't test because of a missed period, I tested because I felt pregnant! Being my third time to the rodeo I had a feeling. I know the exact date we had sex, but every time I put that into calculators it always comes up with me being more pregnant than I am. Can anyone help make sense of this?/help me guess a due date so I can figure out how far I am along until I get to the Drs office? May 31st was the date. Thanks ladies!!