My gut reaction is that she is taking short naps because her A time is very low. Most LOs are onto a 4 hour easy with 2h awake time at 4 months, although yes there are some who take longer. However less than 2 hours at 5 months is REALLY low on the awake time and I am sure she can handle more than that! Especially when you say that it has been going on for a month now, I reckon that when these short naps started she was ready to increase her A time at 4 months.
Starting with your first A time tomorrow, push her to 2 hours awake. You need to extend awake times quite gently, you do not want to go too quickly or she will get overtired (OT) quite fast. If she is showing tired signs before 2 hours I would say that it is habit, and even boredom often makes LOs start with their tired signs! You can make her wind down longer or have some extra time doing calm actiivties until she gets used to the longer A times - but I do think that once she is on more age appropriate A times she will nap much longer for you!