We Brought Our 7 Week Old Daughter. To The ER

We brought our 7 week old daughter. To the ER last night because of a squeaking sound when she would inhale while on her back (it would stop within a minute or two generally). The doctor there thinks it is tracheomalacia. We are wondering if it is normal for this to not appear at all until now and why it didn't happen while at the hospital in spite of having her on her back twice (once to change her and have her vitals taken in triage and again for the doctors exam). She has a slight coigh but it is not frequent but it is beginning to get a bit hoarser sounding when it does occur.