In your case, I think the correct word is pas (Isn't it there a typical sentence of those who like complaining?).
pis comes from the latin pejor, which means worse. In general, if you can say pis, you can replace it by pire (the usual French word for worse). I personally encountered that word in expressions, and occasionally in old books: no one uses it outside of expressions. The grammar guide precises that the word can be used both as an adjective or as a noun.
Reference: Le français correct: Guide pratique des difficultés
Pis (du latin pejus, neutre de pejor) est le comparatif de supériorité de mal: Cela ne va ni mieux ni pis qu'avant (Petit Robert). - Excepté dans certaines expressions, pis est concurrencé par plus mal et par pire.
Pis et pire s'utilisent comme adjectifs ou comme noms. (...)