How did you know she has headaches? Very unusual symptom in a 3 year old! What did she actually say? Have you seen a Paediatrician preferably a Paediatric Neurologist?

Take ur Dr advice

It could be implantation bleeding or a sign of threatened miscarriage. U should go for another scan to make sure all is all and see ur doc. If its threatened miscarriage, u will need to take enough rest, take ur prenatal vitamins, eat healthy and drink enough water, atleast 2.5 - 3 litres a day. All d best!

You don't have to worry. Like your doctor said, the scan result is most likely to be more accurate

Ultrasounds in the early days of pregnancy may not be the best, most accurate gauge of whether you're expecting a boy or girl. Your baby position is also important. If your little one doesn't cooperate during your ultrasound, you may not get that boy or girl determination until he or she arrives

I would expect Scan result when done properly to be more accurate. But remember all results are estimate