As a La Leche League leader for 24 years I have heard this many times.
It is part of the "many mothers have found..." list.  I don't have any
references but I imagine that the natural family planning people might.
Try looking in Sheila Kippley's "Breastfeeding and Natural Family
planning" or other publications of the Couple to Couple League.  I do
tell mothers to check and be sure that they aren't pregnant when this
happens and also to use a non-hormonal method of birth control if they
really do not want to be pregnant. (Religious considerations aside).
Babies who vary a lot in their nursing patterns tend to have this effect
on their mothers more often, particularly when they suddenly increase
night nursings.

Hope this helps.

Sarah

Sarah Friend Barnett   LLLL, IBCLC
Bronx (New York City), NY  -  [log in to unmask]
" You are not obliged to finish the task,
 neither are you free to neglect it."       R. Tarfon