Vampire Strangler.

Lydia, a pretty Romanian woman, arrives in the US and moves in with her cousin Billy. Billy is instantly attracted to her, and it's not long before the two of them are going at it hot and heavy. However, a mysterious shaman suddenly appears and casts a spell on Billy, turning him into a vampire. He focuses his lust for blood and sex on Lydia who, as it turns out, doesn't seem to be all that averse to the idea.