Irregular breathing during sleep.

In the last two months approximately, I noticed my infant (9 mos) has been waking up disturbed and crying a bit every night. She tends to fall back asleep shortly after. Upon closer look, I noticed she does this because she is frustrated with her irregular breathing. She has a mix of shallow breaths and pauses. The pauses are 2-3 seconds duration. But after a while of this she gets very frustrated because she can't breathe and has to catch her breath. I think it happens after she has been asleep for 3 or more hrs. I mentioned to her doctor at 9 month appt but the doctor brushed off my concern saying it is normal. My question is, when does irregular breathing become a problem? How do we know it's not more serious? Thank you for reading