"Obviously" is obviously not the word to use here.
Begin with looking at your pin polygons / texts (whatever
you used to declare connectivity). Right place? Right name?
Maybe more than one VDD pin, one of them on top of VSS?
Though that should give you a label-short warning.
Use the probe tool to select what you know to be VSS and
highlight it. Does the net you know to be VDD get picked up
too? If so, real short.
LVS warnings don't always make literal sense, but they are
trying to tell you something. Sometimes it takes a while of
digging, to figure out what.