From D&D Wiki
 magic Vulnerability
you experience an odd reaction when effected with magic. harmful magic does twice the damage, and is twice as easy to cast on you, but helpful magic has the same result, giving you twice as much health, buffs, ect. at half the difficulty!
Benefit: magic with a helpful effect cast upon the player with this trait is doubled. In addition, the cost of any healing done in towns is cut in half. resurrections are also applicable to this trait.
Drawback: Any harmful magic effect cast upon the player is now twice as potent. Splash damage, area of effect, etc, still affect others normally, but not like the player with this trait.
Special: Potions, enchanted items, and cursed items have a normal effect. Indirect effects of spells still affect the player with the trait. also, spells specifically meant to kill in one hit or reduce the player's health to their bloodied value are treated as normal. Any countdowns related to a cast spell remain the same as well (ie: turning to stone). also, if a spell effects a group rather than just you, the success rate is normal, but you still experience twice the damage. spells like fireballs, summoning monsters, or lightning strikes effect you normally due to it being a produce of magic, not magic itself.
Roleplaying Ideas: perhaps you were experimented on by wizards, or surrounded by magic at an early age.