Quince fruit forming and then falling

Hi,

I have had a beautiful quince tree for about six or seven years. Two seasons ago, it bore a lot of fruit, last season, perhaps because of late frosts, only about three quinces survived. This year, a huge number of quinces have formed, but most are dropping off the tree apparently rotten. There has been heavy rain for the last few days, but perhaps something else is to blame? There are still some fruit apparently developing in the upper branches, so fingers crossed!