Williams only returned to tennis earlier this year after she took a year out of the sport to give birth to her first child.
The former world No 1 reached the Wimbledon final last month but then fell to the heaviest defeat of her career in San Jose last week.
British No 1 Johanna Konta thumped Williams 6-1, 6-0 inside 52 minutes and the American then withdrew from the Rogers Cup a few days later.
The reason for pulling out was unclear until Williams opened up on the difficult emotional time she is currently enduring.
“Last week was not easy for me,” Williams wrote.
“Not only was I accepting some tough personal stuff, but I was in a funk.
“Mostly, I felt like I was not a good mom.
“I read several articles that said postpartum emotions can last up to three years if not dealt with.
The American withdrew from the Rogers Cup two days before the event
“I like communication best. Talking things through with my mom, my sisters, my friends let me know that my feelings are totally normal.
“It’s totally normal to feel I’m not doing enough for my baby.
“We have all been there. I work a lot, I train and I’m trying to be the best athlete I can.
“However, that means although I have been with her every day of her life, I’m not around as much as I would like to be.
Johanna Konta inflicted Williams’ heaviest career defeat last week
“Most of you moms deal with the same thing. Where stay-at-home or working, finding that balance with kids is a true are. You are the true heroes.
“I’m here to say; if you are having a rough day or week – it’s ok – I am, too!!! There’s always tomorrow!”
Former US Open winner Andy Roddick gave his backing to Williams on Twitter.
“You’re a badass lady!” he wrote.