The Dutchman described his fears when he started to feel weak during the Brescia draw on Saturday.
"Five minutes before the end of the first half I started shaking and felt pain in all my body, it was something I had never experienced before," he told De Telegraaf.
"After when I went into the dressing room I passed out and I was very scared. They controlled my blood pressure and heart rate immediately and then I went to Holland for further tests.
"The conclusions were that I am Anaemic and my whole body is very tired. I need plenty of rest."
Sneijder was left out of the squad by coach Rafael Benitez for Wednesday's trip to Lecce.