So here's what I remember exactly:
Spike has been made human without any special Powers, and is with Buffy as a couple. He is kidnapped by Drusilla who wants to turn him again into a vampire. Buffy finds him (with Giles I think) tied up to a bed and dying from blood loss. The person with her tells her it's too late and she has to let him go but she just can't. Because she knows of her slayer demon she tries to «turn» him into a slayer by giving him her blood to drink. It works.
Is it Post-Series (Chosen or Not Fade Away), well it feels like it because I don't think it takes place in Sunnydale, but I'm not 100% sure. The fic was finished (I very rarely read unfinished stories). I started reading Spuffy only 3 years ago so this fic must have still been online.
Someone who seems to remember it told me that Buffy and Spike could be living in England, Buffy could be pregnant and Spike go out at night to get her Ice Cream when he's taken by Drusilla. It indeed rings a bell.
It's driving me nuts not to be able to find it, so any help will be more than welcome.
PS: It's not 'Dangerous' by Mabel Marsters