Another consideration may be this:  With the change in weather, your local
area may be getting an earlier or more prolific bloom of plants that have
toxic pollen.   Chances are that the usual "small population" of the
unfriendly blooms would be ignored by bees in favor of the safe & friendly
ones, when occurring during normal weather conditions.   I have read of only
a few instances where changes in the normal weather have brought about a
bumper crop of toxic flowering plants, so it is unusual.  The ones I read
about occurred in mountainous regions and involved  "milder than usual"
weather for the season.  This "toxic flower" situation may NOT have occured,
but it is not out of the question if your other checks for disease and pests
turns out to show that everything is ok in that department.  Try the pest
checks first.  If nothing turns up there, perhaps a look at the local flora
in bloom would be in order.       - Emily Johnson, Indiana,USA