Maybe she is hungry? 3 weeks is a growth spurt ( and 6 weeks) and I know with Matthew I was always feeding him - but he was breastfeed so not sure if it is any difference? Maybe the ladies who formula fed as well could help...

She isn't cold or hot? Have you tried wearing her? Maybe she just wants comfort? It is hard to guess... it is tough and hang in there. It definitely doesn't last forever.