My baby is 8 weeks old and sometimes she cries for no reason as well. There are multiple reasons as to why an infant cries suddenly.
You can try swaddling her to make her feel secure. Infants may be swaddled until they are 16 weeks old, by when it is better to let them move their limbs freely at night.
http://sg.theasianparent.com/swaddling-101/
http://sg.theasianparent.com/baby-cry-to-sleep-other-alternatives-to-crying-to-sleep/
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