Hello) little one sucking on thumb (does it always mean she's hungry?)…

Hello) little one sucking on thumb (does it always mean she's hungry?), I offer breast - doesn't take, or sucks and cries? tired of crying and going for water to drink. I feel like a terrible mother, don't know what my child needs. Seems like there is milk, maybe not enough? Soon scheduled checkup, seems like gaining weight, don't know. Maybe add formula? Recommend which magnesium B6 do you take while breastfeeding?