High breathing rate in baby. Thoughts.

My baby is 15 months old... From few days my baby is having rapid breathing rate 50 to 55 per min...but this breathing rate occurs only when she is in sitting position...or siting idle...or stnading position...breathing rate is normal when she is sleeping or lying down...29 30 per min... I went to 2 pediatrician both said her chest is clear no infection. ...one ped said ot might be due to nose  blockage... But I am.not getting why does that nose blockage occur while in sitting position only not lying or while sleeping... I vl add here she is having some cold symptoms with sneezing and little wheezing sound while breathing... Baby is active playing well...I want to know what can it be... I am so worried... Also she is havng asd of heart of size 4.5mm... Drs told me.it will get filled on its own....Can it be due to that... Someone told me ask dr abt RSV... Can it be RSV... Plzzz help me dr...I am.getting negative thoughts...