Is this a nest she is guarding?
Yes, I think it must be. Female Dolomedes spiders carry their egg sacs for a while, then build a “nursery web” and hang the sac up in it before the babies hatch. They are very protective mothers and may try to frighten you away if you get too close. Here are a couple of BugGuide.net sightings of Dolomedes mothers, with additional info in the comments:
Carrying egg sac: