Erin,
I also have a (few days shy of) a 15 month old and we had the EXACT SAME PROBLEM (and so do a TON of other moms on this site) - so I conclude that it is VERY developmental as they are getting used to the idea of being a very capable toddler who needs to learn how to get back to sleep with all of their newfound abilities.
With that said, I am going to step out on a limb and say that your schedule looks good. You have about 12 hours from AM to PM and a nap of _________ (how long?). Anyway - I don't think the schedule is as much of a problem as the OTHER stuff.
Are you, have you been doing ANY AP to get LO back to sleep since they started this?
Definitly stop with the bringing her to bed - that will not help her (helps you though)
I would use Gradual Withdrawl to wean her from the YOU that has become the AM comfort and then make a decision on how you will address letting her kow that hse needs to get back to bed.
I posted a long but detailed explanation of the past 2 months with this SAME EXACT PROBLEM at the SAME age!!!!!!!!! I personally do not think this is anything that any of us will "fix" with a schedule tweak. Again, I firmly believe(after living through it - stress living) that it is developmental and the key factor is that we have to teach a TODDLER to resettle themselves in those wee hours when the morning IS so close that they wake and have no idea how to get back to sleep.....it took us 2 months, read my story at the link below.
https://babywhispererforums.com/index.php?topic=86026.0 Stacey