I would just like to agree with what has been said here,as a qualified dental nurse for over 17 years,I would defiantly change your dentist ASAP.It will either fall out over the next few days or be re absorbed by the body.I take it you where given advice on salt mouthwashes as from tomorrow,and not to rinse out today.I would guess it could be many things,non of them that bad,it could even be blood(You know like when you have had a nose bleed)but without seeing it I would not like to say for sure.
If you get that bothered about it,you could always visit some sort of hospital out of hours clinic/walk in centre,just to put your poor mind at ease.
Hope you are OK anyway and change that dentist!
Aya