often do this as they are dying off. though this can happen if they get frosted.

Agree with SBG

Mine too have gone brown, but only from the white camilia - my pink one has not even flowered yet, it's a later variety. But I think most Camilias turn brown after flowering, which is a shame, because unless you pick them off the whole bush looses it's splendour.

We just leave the flowers to drop off and help to compost/mulch the ground.

Camelias apparently don't like early morning sun, so perhaps the position is the problem.

Or overnight frost but I suspect it is simply that they have flowered and the petals are now dying back.