Why does abaqus show negative stresses?

Several things can be going:

1. -ve S22 is actually developing. If you have bending in the model, you can actually get -ve stresses.
2. Negative in the Legend could be just coming from auto limit calculations. Check show min/max in contour options.
3. If you don't expect -ve S22, then check the elements where it is showing -ve stresses in contour. Contour plot is an artistic representation of field variation (to please ignorant viewer). In contour plot, the so called "Nodal Average (75%)" is plotted along with only bottom section point value (-ve to surface normal).
4. To better visualize stresses, turn off nodal averaging (Result->Result Options). Check abaqus manual for how it is calculated (it is very elaborate but quite interesting, you should read it). Also use Quilt plot.
5. Always trust integration values. These are the actual material point calculations (any other are just extrapolation or interpolation).

I hope it helps you in resolving you issue.

------

The world started with 0, is progressing with 0, but doesn't want 0.