Plants can't read labels... The 'normal' time for flowering of that particular rhodo. may well be April/May does not mean it wont flower earlier or later than that. One of our rhodos flowered just before the snow came back - it now has what looks like used teabags all over it. Yes the frost may well kill off the blooms, nothing you can do really just accept it as part of the grand cycle of nature.

"Aye, well, these things happen."
Yorkshire farmer, according to "James Herriot"