The 31-year-old former Destiny's Child member pulled out of the concert on the advise of doctors after suffering from dehydration and exhaustion, reported Daily Mirror.
She was due to perform at the Sportpaleis Arena in Antwerp. Reaching out to her fans in the note she apologised for any disappointment she caused and thanked them for their support.
"To my dearest fans in Antwerp, I have never postponed a show in my life. It was very hard for me. I promise I will make it up very soon. I am sorry if I disappointed you.
It has been reported that Beyonce may be expecting her second child with husband Jay Z.
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She was pictured with a little bump in London last week.
"I would like more children. I think my daughter needs some company. At some point, when it's supposed to happen," Beyonce had earlier said.
She has a daughter Blue Ivy, who was born in January last year.