Blood in my daughters underwear and 6 years old!

It may be possible.  Girls as young as 8 are now getting their periods on a regular basis.  

It could be from something else though.  Possibly an infection, maybe there was some trauma, possibly a hemorrhoid, the list goes on.  Does she have other signs of puberty?

Make an appointment with her pediatrician.  If it is her period there are ways to delay the effects which can often be detrimental to her growth and other development.