Lea-Ann Ellison -- the formerly pregnant woman who was BRUTALIZED on the Internet for lifting heavy weights just 2 weeks before her due date -- is back in the gym after giving birth ... and says both she and her kid are perfectly healthy.

Ellison became an international sensation last year over gym photos -- which showed the 35-year-old doing a crossfit-inspired workout, throwing weights over her head, while sporting a giant preggo belly.

Now, 4 months after giving birth ... Ellison says her son (3rd child) is doing great -- and she weighs LESS NOW than she did before she got pregnant.

Shockingly, Lea-Ann's not happy about being underweight -- saying it just proves she still needs to build up her muscle mass.