Nidah 5

 Unexpected blood 

 Presumably, the blood of Nidah comes according to the (cyclical) Veseth

Nidah is a tumah onto itself.  It declared a tuma in the Torah ( Metzorah). 

There are associated tumoth.  The issue of blood  does not necessarily mean nidah. 

There could be an early miscarriage and that would carry the tumath meith.

There is often ( always) a confusion between Nidah and Zivah. 

Thus, the status of unexpected blood ( Nidah 5) includes these ambiguities.