I don't think there's any 'should' really.
Breastfeeding can certainly affect your fertility and you need to have your cycles happening (at least to some extent) before you can go back to IVF. Some drugs are also dangerous, so there's that to consider.
I would think you'd want to wait a little while, to give your body a bit of a break and concentrate on the baby. I suspect most FS would probably want you to get to 6months post-partum or something like that before going again.
I was too buggered to even contemplate it till DS was 18 months...