Fun Fact: The protagonists name is the same as the vampire Matilda in the 19th century novel 'Matilda' which is a central piece in the vampire genre as a whole and also has clear Sapphic themes
I think you are referring to the 1872 Gothic novella
Carmilla by Sheridan Le Fanu, which is, as you described, (one of?) the first instances of vampire fiction, and inspired the more famous
Dracula novel in 1897. It is very good, and a short read. It's old so anyone can read it for free. I recommend it.
Mathilda is a is a 1820 novella by Mary Shelly (author of Frankenstein) and it's about incest.