Zsa Zsa Gabor was rushed to the ER Monday ... 2 days after her 99th birthday, TMZ has learned.

Gabor was having difficulty breathing ... husband Frederic von Anhalt tells us.

Zsa Zsa's doctor came to her home and removed mucus from her throat, but it didn't cure the problem. Zsa Zsa was admitted to an L.A. hospital Monday morning.

Docs determined she has a feeding tube-related lung infection.

We're told Zsa Zsa's scheduled to undergo surgery to have her breathing tube removed.

Gabor has made frequent trips to the hospital over the last decade. Her right leg was amputated in 2011.