Then the woman added: "that the men at the pool would feel more comfortable if I was in a one-piece swim suit because a bikini isn't appropriate for a mother, especially one who's 'still recovering."
Ms Sinclair wrote: "Anyone who knows me knows I had a few words for this crazy, ballsy woman. But instead I just smiled, kissed my son and told her, 'I'm proud of my body. In just one year I've gained 50 pounds, and lost 37. I've grown a human and given birth to a beautiful miracle. My body provided food for my child. So, no, my body might not be the best sight for other men to see. My stretch marks and tummy pudge might not be sexy. But they're proof that I've done something amazing, and I have a man that loves me and finds me even sexier and more beautiful now. To be honest, I don't give a tiny rats a** what other men, or a prissy twit like you think. Have a nice day though."
The post has been shared 23,000 times.
People have written supportive comments to tell her she did the right thing.
One person wrote: "You look damn good for having a baby 4 months ago. I hate how so many people body shame mothers. We literally have done the most amazing thing.
"We brought innocent life into the world. We held a child INSIDE of us for nearly a year. We changed our lives to ensure the life we were creating would have no harm.
"We did something that NO man can do. Why would I try and help a MAN "feel more comfortable" with my body, when they know nothing in the art of creating life.
"They don't know the pain, the sickness, the body changes. Nothing. And for woman like her, just because she may be ashamed of her body after pregnancy, doesn't mean you or anyone else should."
Another said: "Well that's extremely rude for someone to say to anyone. Don't let people get to you. You got your boy that's all that matters!"