*hugs*
I'm having the same issues with my DS. He was 2 weeks early, and he is now 18 weeks. I have him on 2 hours and 15 min of A time now, and that seems to help. I was putting him in his swing for the last 15 min and just watching to make sure he didn't fall asleep there. It was his quiet wind-down time. Then I discovered that he actually slept better if I was interacting with him towards the end of the A time. I thought I might be overstimulating him, but that wasn't the case. I guess you just have to find what works for your LO, but I would suggest trying to extend her A time and keep it consistent. She will get used to it.
What are you doing during her A time? She may need more, or less stimulation as well. It's such a fine line!