Dates

Hey everyone, I found out today I am pregnant I have suffered losses and an ectopic previously. I'm told I need to have a scan at 6 weeks now my question is my last period was 4th july but it was very light and only lasted a day, I'm concerned that I may have my dates wrong and my doctor is calling me tomorrow to discuss this so do I just say my last period was the 4th or do I explain that I could be further a long? I'm a nervous wreck about the whole situation and can not sleep for worry