Can’t get back into sleep routine

My 15 month old for the last 3 weeks straight has been outright fighting bedtime till 10/11pm at night, and then refusing to wake until 9/10am. I will even get her out of bed and start housework ect and she will just refuse breakfast & hope on the couch and go back to sleep, then sleep the car ride to school.

I don’t know where this has come from. I have a 6 year old and we religiously have always had a 8pm bed time and 7am wake time. Some days she wants a morning nap and afternoon nap and other days she hangs out and has 1 biggish afternoon nap, without issue.

It’s driving me insane, we still breastfeed and co sleep, but the only way she’s going to sleep at night is in the pram then I move her to the bed for the remainder of the night once she’s asleep, but it’s a fight until she finally dozes off, and by that point some night I give her pamol. She’s always cut teeth pretty quietly without a fuss so sometimes I just guess it could be teething pain..

Im go back to work very soon and she will be staying home with family until I feel ready for daycare and I’m so stressed about being tired for work bc normally our early bedtimes compensate for her waking a couple times a night to eat /soothe but now I’m not getting those extra hours and still feeding overnight, I’m so exhausted.

Any tips?