As baby is too small.. they don't know more about day and night.  Crying may be for hunger or colic pain, gastric, cold...can be for anything. Have to bear and try to make baby calm. It's all new part of motherhood .

if your milk supply is proper or not.. Check the post on my page.. Its about how to put babies to sleep.. Might help !!

Babies do that a lot till atleast 3 months.. Also check