It really sounds like a flea you saw - look on the tummy area - groin area and top of the head too - oh and also under her chin -
they're fast little blood suckas and you may need to get some frontline asap before they get worse. My girls only had them one time - and I only saw 2 fleas - but man were they scratching......
another way to help her is to bathe her in Dawn Dish Detergent - watch the water for any dead fleas - then wait a day and apply the topical treatment.
You may only see one flea - but where there is one - there's more.
Also treat her bed etc so you don't end up with an infestation. With the cooler weather coming you may be ok but try to take action before it gets worse - she'll be miserable if you don't treat her
good luck ...i hate fleas - they freak me out.