Yeeeeaaaah!!!! It also hints it quite a lot. ;)
1) In The Winter Soldier, Nat and Steve are talking about going after the mysterious WS and Nat says, "Going after him is a dead end. I know, I've tried." She's tried to find him.
2) The first fight with TWS (on the bridge), you see Nat running away from Bucky and when she looks back, you can TOTALLY see a smirk on her face. She ain't even the slightest bit afraid because she totally knows this boy.
3) In the comics, it is explained that Natasha was trained by Bucky in the Red Room. If you carefully observe their fighting patterns in the movies, they are incredibly similar.
4) In Civil War, after Zemo brainwashes Bucky, Nat gets a turn fighting him and says, "You could at least recognize me." O_O
5) Two of Russia's posters for Civil War showed two characters face to face. Wanda and Vision. And Bucky and Nat. #Russia'sShippingGameOnPoint
6) When asked about Winterwidow, Sebastian Stan has said he wants it to happen. *squeals*
7) When Bucky and Steve are suiting up before their stand-off with Tony, Bucky pulls a gun from a rack. Guess what the rack says on the side? "Romanoff"
ITS HAPPENING. FIGHT ME.
Besides, how can you resist all the adorable Pinterest fanatic???