1. There may or may not be an identifiable explanation for spotting during the luteal phase.

2. Specialists argue about whether or not it affects your fertility.

3. Plenty of people (including your Great-Aunt's friend's coworker) have spotted heavily during their luteal phases, in some cases losing up to 15L of blood in one day and requiring multiple transfusions, and have soon afterwards found out they were pregnant and gone on to deliver a healthy baby. Even though multiple doctors had told her she would never be able to conceive on account of having no ovaries, plus she hadn't had sex in about six years and in fact had been living as a hermit-nun on a deserted island since her husband died five years ago, and in THOSE days we didn't have access to all these fancy assisted reproduction technologies, we had to make our own fun.

4. Here is a somewhat random list of terms for you to ask Dr Google about, and later regret doing so:

• anovulatory spotting (fixable by inducing ovulation)
  
• ovulatory spotting (more mysterious and complicated)
  
• coagulopathies
  
• structural lesions
  
• hypothyroidism
  
• inflammatory lesion
  
• dysfunctional uterine bleeding owing to immaturity of the hypothalamic-pituitary-ovarian axis* (to which I would add thoroughly-confused-by-infertility-drugs hypothalamic-pituitary-ovarian axis)
  *reproductive hormone system, in plain English.
  
• endocrinologic causes (hormonal problems) vs organic causes (non-hormonal problems)
  
• organ dysfunction, such as liver or renal disease
  
• endometrial hyperplasia
  
• infection
  
• iatrogenic causes (caused by medical intervention), such as chemotherapy, anticoagulants, steroid therapy, and use of IUD
  
• anatomic causes, which include uterine leiomyoma, endometrial polyps
  
• endometriosis
  

• hormone tests - luteal phase progesterone, serum prolactin, hell, haven't we all had enough hormone tests done to cover this bit?
  
• hysteroscopy/hysterosalpingogram/sonohystogram
  
• thyroid tests
  
• adrenal function tests
  
• laparoscopy
  
• endometrial biopsy
  
• coagulation profile