So how do I know when I got pregnant

I have had this same issue, however what I have been told by the doctors is that it is just a guesstimation. You can ovulate without having menstration. The doctor should be able to give you more of a definitive answer depending on your symptoms and size of the baby which they can tell by an ultrasound.

Even if you know exactly when your last period is, they only estimate ( give or take a couple weeks) So when you do ultrasound they will give you a estimate.

But the exact time really does not matter unless there was sex with different partners