The Virgin Mary & Labour Pains...

No, I'm not inviting this debate over to my comments box, thank you very much. ;)
Rather I came across this from one of John Paul II's audiences:

On Calvary, Mary united herself to the sacrifice of her Son and made her own maternal contribution to the work of salvation, which took the form of labour pains, the birth of the new humanity.

I half-remember encountering this idea (Mary suffering labour pains at the foot of the cross) somewhere else, perhaps in a patristic source or a breviary reading. Can anyone point me to something like that?
Update
Thanks to Fr Vidrine who gives a citation of St Bonaventure in my comments box:

I think you're referring to this reference of St. Bonaventure: “That which in the Nativity She brought forth with joy, in the Passion She gave birth to with sorrow” (Commentary on Luke, c. 23). Here he joins the mystery of Our Lady’s suffering at the foot of the Cross with the mystery of Our Lord's birth at Christmas. Our Lady does not experience the pains of childbirth at the Inn of Bethelehem, but at the foot of the cross as Mother of the Church.

I've not read a huge amount of St Bonaventure, so I doubt that it's that text I'm remembering, but at least that reference establishes the scolastic (if not patristic) credentials of the idea.
I've also stumbled across this article in an art magazine (which I've only started to read) called The pain of Compassio: Mary's labor at the foot of the cross by Amy Neff. Please bear in mind that it's an art history article, so I wouldn't expect the theological precision of a theology article, but on page 18 it seems to provide some useful citations for the idea in question:

28. Albertus Magnus, Postilla super Isaiam, 110.49; my translation, from Albert Fries, Die Gedanken des Heiligen Albertus Magnus uber die Gottesmutter (Freiburg, Switz.: Paulusverlag, 1958), 151. For other texts in which Albertus Magnus repeats similar ideas, see Fries, 10, 184, 220, 321, 335-38; and idem, Was Albertus Magnus von Maria sagt (Cologne: Amerikanish-Ungarischer, 1962), 11 6-25. Fries notes that the conceit of Nature exacting payment from Mary was derived from the writings of John of Damascus. It also seems to be connected to the idea, presented in Alain de Lille's De planctu Naturae, that man's birth is through the goddess Natura, his rebirth through God. A concise summary of Alain's concept is in E. R. Curtius, European Literature and the Latin Middle Ages, trans. E. R. Trask, 2d ed. (New York: Harper & Row, 1963), 118-19.

29. Saint Bonaventura, from the chapter on Fortitude in the Collationes de septem donis Spiritus Sancti, in Opera omnia, vol. 5 (Quaracchi: Typographia Collegii S. Bonaventura, 1891), 487a; quoted in Emma Therese Healy, Woman according to Saint Bonaventure (New York: Georgian Press, 1956), 239-41.

And there's lots more.... I guess I'm going to read the whole thing when I get a chance and see if I can't recall the text (was it St John Damascene?) where I picked up the idea originally.
Bingo!
Neff quotes Rupert of Deutz:

At the foot of the cross, Mary] is truly a woman and truly a mother and at this hour, she truly suffers the pains of childbirth. When [Jesus] was born, she did not suffer like other mothers: now, however, she suffers, she is tormented and full of sorrow, because her hour has come. . . . in the Passion of her only Son, the Blessed Virgin gave birth to the salvation of all mankind: in effect, she is the mother of all mankind.

with the following bibliographical detail:

21. Rupert of Deutz, Commentaria in Evangelium Sancti Iohannis, ed. Rhabanus Haacke, O.S.B., Corpus Christianorum: Continuatio Mediaevalis, vol. 9 (Turnhout: Typographi Brepols Editores Pontificii, 1969), 743-44; my translation, for which I also made use of the French trans. in Therel, 125.

I'm pretty sure I've seen that quotation from Rupert referenced in some theology article I read about Mary's motherhood of the Church.
It's worthwhile chasing down the pictures she refers to using google. Here, for example is a late 15th century crucifixion by Rueland Frueauf the Younger where Our Lady is seemingly depicted on a birthing-stool at the foot of the cross.