Just another voice here to say it's very unlikely to get a 2-3 weeks pregnant result 7 days after sex.
It takes 6-10 days for a fertilised egg to implant in the uterus, so that's 6-10 days after sex before the thing's even attached to you and producing hormone, and the amount it produces at the beginning is absolutely negligible anyway.
Like Hopeful I have spent a lot of time taking pregnancy tests, many of them positive, and I have never had a solid positive result until at least 12-14 days after fertilisation.
The fact that you had symptoms also points towards a longer-standing pregnancy, as they are caused by rising pregnancy hormone levels.
I also find CB are not particularly sensitive, so to be getting a 2-3 I think you're too far along for it to be your friend's.
I have also had a number of early scans, and as someone has said before two weeks' difference in embryo size is very apparent. You can have a private viability scan at about 7 weeks if you want to, and they will measure the embryo and give you from this an estimate of how pregnant you are. It's not hugely accurate but it is accurate enough to tell the difference between August 25th and Sept 7th. You can look up online places that do these scans, and they're done perfectly privately. They take about half an hour and cost around the £100 mark.
Perhaps it would help set your mind at rest if you had an early scan and got the dates sorted. Is there someone you could take along with you?
I hope this all works out for you, it sounds very stressful. Don't beat yourself up about being 'careless' and 'silly', we're all human and we all just make the best decision we can at the time. That's life for you
