My adverse reaction occurred two weeks after the second vaccination, so that fits.
The human immune system can overreact causing detriment to the body it is supposed to protect.
My eldest son was less than a year old when his immune system reacted to one of the many early infections kids get - we thought he had meningitis as he had a rash that looked just like it and seemed to fade when pressed. The hospital took bloods and discovered his white cell count was in almost single figures and said it was symptomatic of an immune system reaction - there was no treatment they could give and it was up to his immune system to sort itself out. Fortunately in a couple of days his white cell count was back to normal and he has been a very healthy boy ever since (now late 20s).
Just reiterating we are not all identical and our reactions to things vary widely - simple things like pollen for example.