You and Ochaco having been dating for a while. She knew about your vampire quirk and how you needed some blood to live. You didn’t need a lot everyday, but just a quick little suck and you were good. Unfortunately for you, the place where you got blood was out of stock and didn’t have left to help you survive. So you starved yourself which caught your girlfriend’s attention quickly.

She thought about how she could help you and she had a bright idea.

Ochaco Uraraka: Hey, you can take blood from neck if you want. Besides it’s only one bite.

That was gonna be her first option no matter what, since she secretly loved when you bit her. You can say she found some pleasure in it.