Lea05 - why do you need the undies ?
Unless you have a male - at home - with all his bits - I would not bother with the undies - just keep her within eyesight and on lead.
With my girls - me - trained by my vet at the different times - they showed me how to work out when the girls would welcome the boys. It is not bombproof - but gave me some idea and time !
Place your fingers at the end of the tail where it meets her body - either side does not matter - if the tail wags to either side - then she must be in total lockdown.
As far as desexing her within 2 weeks after her bleeding stops - well - maybe your vet is not as good as I thought. She will need a lot more time - than just 2 weeks after the bleeding has stopped. Us girls know all about hormones - but with us it is every month - girl pups - not so.
You now know her cycle - sort of - so I would be waiting longer to desex your girl than what your vet has suggested. You need her hormones to be settled - before you desex her.