Blake Lively has told people there were multiple reasons Justin Baldoni made her feel uncomfortable on the set of "It Ends With Us" ... TMZ has learned.

Sources with direct knowledge tell TMZ ... there was a scene in which Justin lifts Blake into the air. According to our sources, Justin has a history of back problems and before lifting Blake, he went to his on-set trainer and asked how much she weighed and how could he train to protect his back from injury.

Blake later found out about Justin's comment and felt he fat-shamed her.

We're told this comment really bothered Blake, who gave birth to her and husband, Ryan Reynolds' 4th child in February 2023.

Our sources tell us the second incident that allegedly made Blake uncomfortable on set is when the two engaged in a kissing scene and Blake felt he lingered longer than he should have with the kiss.

We broke the story ... cast and crew agree there's definitely bad blood between the "It Ends With Us" costars -- but sources told us Justin's not the clear-cut villain, as some people who worked on the film are standing by him.

Some sources we spoke to also claimed some cast members are milking the drama to market the film.