Toddler always tired - should I be worried?

[WineGetsMeThroughIt]

I used to be really pleased that my daughter was such a good sleeper. She'd sleep from 8pm-11am (or even 12-2pm!) the next day on a consistent basis (going on 2.5yrs). She's never woken before 8am. My son slept until 11am for over a year and a half as well so I just found this sleep pattern for both of mine normal.

Recently though I've become a little worried about my daughter. Unless there's something really worth her while like other children to play with, or she's at nursery shes often really tired and just content to lay on the sofa. She wakes up at 11am after 15-16hrs of sleep and before I even get her out of bed she's saying she's tired. When I try and wake her up for nursery at 8:30 she's like a teenager trying to ignore me and turn away and go back to sleep. She loves going in her stroller and falling asleep.

She's kind of always done this which is why I've never really been worried, but now I'm wondering if I should I be worried?

I've tried making an appointment with her GP just to check but they're closed today so I'll have to wait, but just wanted to get others opinions.

Some children are just sleep-loving but this sounds pretty extreme. My DC never slept much and rarely sat still at that age. Everyone's different of course but I would definitely speak to GP. I know lots of children that sleep and nap extremely well but not wanting to do much when awake sounds unusual.
What's her diet like? Drinking ok? Weight/growth normal? Developmentally normal? Do you do you best to make things interesting/stimulating for her when she's awake?

It's hard to know. My close friends daughter was sleeping 7pm-9am and with a 3 hour nap at that age. This was shocking to me as my boys are the total opposite. She is 4 now and has dropped the nap but still sleeps 14/15 hours a night. She is being assessed for ADHD but otherwise seems typical.