Oh dear. It must feel like your walking on thin ice. Has she always been this way? If no, can you recall when she started picking on you?
you first need to gather all courage AND patience. Ask her what she's truly unhappy about. Did she recently give birth? Is she a stay home mother? It'd be good if you can provide more background details so we understand better.
Personally, I do quarrel with hubby over trivial matter cos I'm only human and humans are not perfect. We all have our up days and down days. But what's important is communication in a marriage. No communication, marriage will spiral downwards eventually.