White flowers, red flowers, yellow flowers. Flowers our speaker doesn't like anymore, flowers she does seem to like (albeit in a creepy, suicidal way). Why so many flowers? Of course it has something to do with the fact that it's springtime outside. Inside, however, it's grief-time, and those flowers are not helping matters.
Line 9: Those white flowers are just bursting with potential symbolism. First off, the image contains within it all the associations of spring. After all, flowers are kind of spring's forte. The color (white) has its own associations, too. On the one hand, it can represent purity, and we might think of the white dress of a bride, and even her wedding cake. From there, though, we think of marriage, and husbands, and suddenly (because the speaker of this poem is a widow) we think of death. And while we're there, we can note all the associations white has with death: bone-white, ghost-white, the white light at the end of the tunnel, etc. No matter which way you look at it, it seems, this poem leads to death.
Line 10: That phrase "masses of flowers" is a bit of a hyperbole. That is, we doubt that there are really enough flowers to qualify as "masses," but they weigh down our speaker emotionally, and she exaggerates the amount of them in order to convey that.
Line 11: The poem begins this line with the last three words of the line right above it. This repetition helps convey the weight of the image.