Who knows. It was just said that her mother was from a race of magical sorcerors who came from Atlantis. Her father was also a decendant of other magicians. Shes naturally knows magic via born with it. Her father taught her how to concentrate and hone her skils.
Where does it come from exactly? I dont know. I guess more from her mothers side.
The reason I was interested in that is because if it only comes from her own mortal, innate bloodline, it puts her verrrrrrrrry low on the totem poll of the universal magic Hierarchy.
If it's only from her, then negating her spells by simply just BEING Dr.Strange, Sorcerer Supreme, Champion of the Vishanti, is very plausible.