From the article:
Breastfeeding agitation is no fun, and pregnancy seems to be a particularly common time for breastfeeding agitation to strike (affecting roughly one-third of pregnant and breastfeeding mothers). Some pregnant mothers can tell you exactly what week the agitation set in for them, and although it differs from woman to woman, mid-pregnancy seems to be a common time of onset. Many women find that the agitation abates in late pregnancy, while others find that this is when it’s most intense.
Still, agitation can strike any mother who is nursing an older child, even without a new pregnancy. During tandem nursing it tends to be triggered only by the older child’s suckling, or it may happen only when both children nurse simultaneously. Tandem nursing mothers who subsequently become pregnant may be particularly likely to get it. In many cases the trigger may have little to do with pregnancy or tandem nursing.