Miscalculated weeks!

Kandy • 43 year old TTC... Keeping the Faith!
 Hi ladies! Maybe you can help me! I'm a bit confused because I was under the impression that I was 7 weeks (my LMP was 2/17) however I just came back from my <a href="https://glowing.com/glow-fertility-program">IVF</a> Doc and she says when you do <a href="https://glowing.com/glow-fertility-program">IVF</a> your pregnancy is calculated differently. She believes I had a late implantation. Because Last week Wednesday night I was spotting very lightly and it lasted for a few, then in Thursday I wiped again and spotting was brown and very light, it also lasted for a few - it has now stopped completely. 
I just came back from the doctor. They did blood work to check my HCG Level ( I will get the result later) and according to the ultrasound I'm 5 weeks and 2 days! She did see something in the ultrasound however she says is very early to tell - has anyone experienced anything like this? BTW I took a HPT and in less than a minute I got this BFP
Has anyone miscalculated before or done <a href="https://glowing.com/glow-fertility-program">IVF</a>?