Worrying about new arrival in November as my 2 year old has started playing up 😢😢

I have a 21 month old daughter and I am due my 2nd baby in November! I keep freaking out about not being able to cope! Especially because my daughter is starting to play up! She has always been very good. She was always a good sleeper, she used to go into her cot and just go off to sleep herself no problems. But lately that's changed! Last few nights we put her to bed as usual but she scream and screams to get out of her cot. Tonight I let her cry it out. She cried for 20 minutes but eventually fell asleep. She been doing the same at nap time! I'm hoping this is just a short phase and she will go back to normal! I'm trying to be very firm and not give into her and show her that she will not get her own way! I'm so worried about the new baby waking her in the night or vice versa! I'm never going to get any sleep ever again! I'm totally freaking out! 😱😢