Old-fashioned Bleeding-heart (Lamprocapnos spectabilis, formerly named Dicentra spectabilis) has a white-flowered form called 'Alba', which has been in the horticultural trade for many years. If you planted any Bleeding-heart that weren't yet old enough to flower (or which had already finished flowering when you got them), this one could have been mislabeled and mixed-in with the pink plants. Otherwise, it is a natural mutation that may have sprung-up on its own among seedlings if your plants have propagated themselves.

Miri