8 month old sleep

Please help! I’m at my wits end. My 8 month old daughter has suddenly decided she will only sleep at night if she’s being held. The past 3 days she has screamed all day, she’s fed, clean, had stuff for her teeth incase that’s what is wrong and she will fall asleep at night and as soon as she’s put in the cot she’ll wake up and scream. I don’t know what else to do. I can’t sit and hold her all day and night as I have a 2.5 year old aswell. I’m so tired I could cry.