It says in Breaking Dawn that she died in a car crash....
'' A car accident, like my mom. So common. Happened all the time'' (Page 149)
and ''It's hard, the girls where older than you when you're mom passed. They have more trouble being in this house'' (Page 166)
so we can safely assume that she died in a car crash when Jacob was quite young.
Also, I'm pretty sure that it wasn't werewolf related as Billy was never a werewolf (it says somewhere that it skipped Billy's generation, but despite that he still have the ring of authority like an alpha or something) and there were no werewolves around after Levi/ Ephriam (?) until Sam

converted werewolf girl.
Imprint me?
Not a drug, I would've been the air, the sun.