Large negative values of pressure indicate that cativation could occur here. Overall the model may be very good, but in this small region where the "negative" pressure occurs, the model is not going to be very reliable. Nature would account for this negative pressure by having the liquid transition into a vapor. Since you are not including that in your model (it is possible to do), your model will not be accurate here.

Cativation can be a very big problem with pumps that can lead to failures. For full disclosure, when documenting the model, you should point out that cavitation could be occuring there.