I wonder if it's like when you avoid getting sick for months of working hard and surviving on high stress and minimal sleep, then you go on holidays and get sick. Your immune system 'holds on' for ages then kind of gives in when you relax. Maybe your body experiences breastfeeding as increased demand and ramps things up to cope... Then let's go when you wean?

I'm having a similar experience in that until two weeks ago I was extremely low in iron, working part time and kid-wrangling two littlies and in spite of them having been sick 6 times this winter, I've survived with only the occasional sniffle which my body has quickly fought off. Then in the space of a week I finished work and had an iron infusion. I should be feeling heaps better, but instead, wham. Sore throat, aching muscles and a cough. Why??!

Ok, sorry, that turned into a whinge too but might explain your experience?