Raspberry leaf tea is claimed to soften cervix, but it’s very much a theory and not a fact. Perhaps this could be a reason or maybe it’s from sex or baby pushing down in the cervix and old blood is coming out? Have they talked to you about that/asked those questions?
I dont have any experience about this but my midwife suggested not taking raspberry leaf tea until week 36 as its purpose is to prepare the body for birth. She said taking it too early can induce an early birth for some people