Aim of the study: Early motherhood is the time when the female body must cope with sleep deprivation, fatigue, and stress associated with increased responsibilities following the birth of the baby. The aim of this paper was to investigate the psychological wellbeing and physical activity of healthy mothers in the first months postpartum according to the infants’ feeding method. Materials and methods: 24 healthy exclusively breastfeeding and 13 healthy exclusively formula-feeding mothers participated in this study. The results were based on the standardised scales: Fatigue Severity Scale (FSS), Epworth Sleepiness Scale (ESS), Perceived Stress Scale (PSS), and Kaiser Physical Activity Survey (KPAS). Results: When comparing data between the two studied groups, we found that breastfeeding women reported lower levels of sleepiness (p = 0.011) and fatigue (p = 0.0006) than formula-feeding mothers. Perceived stress and physical activity did not differ between the groups. Positive correlation between PSS and FSS was found (r = 0.62, p < 0.005) in the group of breastfeeding mothers. Conclusions: 1) In breastfeeding mothers, fatigue and perceived stress reinforce each other. This relationship serves as an alarm signal to provide nursing mothers with optimal care in order to prevent them from stopping breastfeeding. 2) In a healthy population of mothers from 3 to 6 months postpartum, exclusively breastfeeding women are observed to have lower fatigue and sleepiness levels compared to those who decided to feed their children with formula only. These conclusions may be an additional source of information for the promotion of breastfeeding.