Don’t know my due date! Going based off what I think is the conception date!

Hello ladies!

I just found out I’m pregnant with my 3rd baby but have no clue what when I’m due. I haven’t had a period since I gave birth to my baby girl 8 months ago so I’m just going based off what I think is the conception date but I could be COMPLETELY wrong. My OB said I should still wait the 4-6 weeks to get my first ultrasound so my appointment is September 4th but I genuinely feel HUGE! Like my stomach looks 5 months pregnant lol. Could be the extreme bloating but that’s really all the symptoms I have other than super tired. But could be from my 2 year s and 8month old lol. Anyone experienced not knowing your due date? What did you do? How did you not feel so anxious waiting for your appointment??