You are 19 yrs old, obese
and mother of 1 yr old child. You say that your resting pulse rate is 90/mt. While you chase your son you become short of breath, develop a feeling of pressure over the chest. Because of this you say that your pulse rate is 60 or did you count while you chase.
Even chasing is a mild exercise - so the muscle needs more O2. So actually the " "pulse rate has to increase". Short of breath and feeling of pressure over the chest are due to increased O2 demand.
Try to reduce obesity
Investigations to rule out any particular cause for obesity (hypothyroidism
This will clear your doubts.