It could be UT, yes. The sign is the waking at 45 mins really. Is she waking up happy?
Have you looked at the average A times here?
Average A times- BOOKMARK ME! An average 6-7 month old is going to need to be awake for between 2.5 and 3.5 hours between sleeps. Those A times in your day are only just two hours each, so are short even for a 6 month old. I definitely think stretching the A times will help. Also, I know a slightly premature baby will take a little bit of catching up, but by this stage, and having been 4 week premature she may well have caught up to her actual birth age, without need for adjustment.
If you take the A time extension slowly you'll be able to see when the A time is long enough, without worrying about her getting OT.