Assuming that the stain came from a crushed "Maacholet" bug: Niddah 19b, 58b
Assuming that the stain came from her husband or child: Niddah 19b, 58b
Assuming that the stain came from a medical plaster: Niddah 19b-20a, 58b
Assuming that the stain came from tree sap: Niddah 20a, 58b
Underage Women
The bloodstain of a woman who is underage for menstrual bleeding: Niddah 10b
What if a woman is underage, and experiences bleeding with 90-day intervals, and then finds bloodstains during the intervals: Niddah 10a-b