It's not exactly a surprise that a poem that calls itself a "Lament" and has "Sorrow" as its first word deals with sadness. The force of our speaker's sadness, the ways it acts on her, is what "The Widow's Lament in Springtime" is all about: the cold weight of sorrow, the way it transforms joy into distance, and how it isolates the speaker. Need we say more? Nah, we'll stop there. You'll just have to read it to find out.
This speaker is not just sad about her husband. She is also totally bummed that she has lost her connection to the natural world.
The speaker's sorrow comes from the fact that the world around her isn't grieving with her. She's all the sadder because everything else is in bloom.