Yeah, I agree - I think it's about two vampires. But, it's also about how strong love can be, it makes you 'so weak inside'. She's referring to the fact that she misses her love so much, she feels so weak. Then, when she kisses him, she feels so alive again and feels like she can live forever. But she's also saying that her love is deadly, so she's giving him a chance to leave before it's too late.