It is not usual for a 3 1/2-month-old infant to keep her hands fisted most of the time. This behavior, which reflects neurological maturation, is more typical of a younger child. So unless your child was born prematurely and has an adjusted age (that's her chronological age corrected for her prematurity) of 2 months or less, this behavior needs a second look. It's time for her 4-month checkup, so be sure to keep that appointment and point out that behavior to her health care provider. Additional evaluations may be needed. Stress doesn't show up like this in infants so I don't think it's the explanation for your baby's behavior.