Grass Issue?

[bigdog]

I have an issue with a section of my grass, after this past winter season.
The grass in question was fine last Fall, no issues. The patch of grass in question, is roughly 16x10, and appears spongy in some places when walking on.

There are no holes or tunnels on the surface, at first I thought it may be grubs,
or maybe just a nitrogen deficiency, what's your thoughts and opinions.

Thanks.