I've been looking for this fic for what seems like forever. Natasha and Steve are looking for the Winter Soldier, when they find him Natasha tranqs him but instead of putting him out it drugs him up and he becomes clingy towards Steve and jealous of Natasha. He keeps calling Steve a bunny in Russian. I'm sure there may have been more to the fic but that's the part that stuck out. I read it on Ao3.