If you're immune, your immune system should kill the rubella before it comes into contact with the baby, but it's still best to avoid it. It's not a dumb question - you're immune, but the baby isn't, so I'd still suggest caution (as do The Rules over here in the UK). However, as you need to contract rubella before the foetus can have it, your immunity is important.

I'm not sure if the disease will pass into the breast milk, though the antibodies will, so having the jab whilst BFing should be OK, but don't quote me on that! Isn't the shot the dead rubella anyway?