hi coleymail and welcome to the boards!
a few questions and we'll see if i can help at all:
she is 5.5 months old now?
does she wake up after 45 min or are you waking her up?
does she go to sleep independently?
any props?
has she ever had long naps?
how many NWs? only the one for a feed?
if she is 5.5 months old she seems a bit low on daytime sleep so i think she is OT which can cause all kinds of problems incouding short naps and NWs...but answer my questions and i'll get back to you.