Alright, for starters, sulfur itself reacts with water, bubbling would be normal. (Also, wicca is not evil so I don't see how holy water would effect it at all

)It MAY have not been sulfur at all, a lot of things smell like sulfur, so it still could have been something else. My suggestion would have been not to touch it, not to put water on it and to save some to be tested by the local high school or collage. Sulfur in wiccan spells is used as a base, not as an evil agent, and when used over doorsteps, its a protective element, just like salt. Of coarse I don't know what the implements of sulfur are in VooDoo but they are likely the same. Salt is a protective agent in wicca
and voodoo, its meant to keep evil spirits out.. this can also be done with brick dust. This is a protection spell, as using wicca to harm another is a big no no and really just plane stupid. So the likelihood of someone placing a demon in your home then using the salt or sulfur to keep it in is unlikely.. also.. as soon as the salt or sulfur was removed from the doorstep, it stopped working. So unlikely and ineffective.
Grave Dirt is used as an agent for summoning spirits and protection. Some people claim it can be used for love spells, though I've never believed in love spells, you can't force someone to love you.. maybe notice you though. Such a spell might or might not use grave dirt. Disregard the love spell use.. you have spirits and protection. Using grave dirt to summon a spirit is more for communication then actual haunting, and go GET grave dirt that would work for a spell requires another spell to be preformed on the grave itself, and a favor to be granted to the owner of the dirt (the dead person). Normally this takes weeks to complete and once grave dirt is acquired it is used very sparingly. If it was used to summon a spirit you would have had no more then a pinch, and it would be useless just left there, as once the spell is done the spirit returns to its rest.
Grave dirt for protection is gathered through the same process and is normally held in a pouch carried on the person, as it is used sparingly. dropping a load on a doorstep would have protective ability's, if the right spell was preformed, but again it wouldn't be more then a pinch or a very small handful.
Normal dirt is used in spells, but these are healing or growing spells, not even for protection.
In the end I think maybe your looking at something totally mundane and seeing something sinister, and those thoughts alone can be enough to bring on back luck, bad dreams, sadness.. the mind is a powerful thing. I see no reason what so ever for approaching anyones girlfriend about being a witch and casting spells, you might just make someone mad and have some very real and none magikal problems to deal with. Wicca says "if it harm none, do what you will" (yes I know but I hate old English >.<) so if its a witch trying to cast a harming spell, they're doing it wrong, and STILL going to be punished for trying to hurt someone.
The other aspect is voodoo and I know very little of the religion, you don't live in Louisiana, Mississippi or Florida do you? You would have to talk to someone who knows more about it, but I really don't see why anyone would be bothering you either way.
Good luck, and hope that helped some!!