Boobs itch when im not wearing a bra

Okay well i am currently 19 weeks pregnant and my boobs have gotten bigger. Whenever i am not wearing a bra for a while they start itching, around the areas where you usually get stretch marks. Could it be the weight is too much for my skin right now? Because when i wear a bra i am perfectly fine, no itching. And i put lotion all the time so it can't be that I'm dry