by your discription it seems you are talking about respiratory rate
not heart rate
. so there can be number of thing which can cause increase in respiratory rate. most of the benign candition should be resolved within 48 hours, meanwhile other significant condition should be looked for like cardiac cause which need cardiac eco, pulmonary cause which can be ruled out by simple x ray
chest. infection can increase rate, septic screen needed to rule out this , once screen is negative antibiotic can be stoped.