Mom's of two or more

My 2 year old is acting out a lot since baby came home. I feel like all I do is yell at her. She's doing things she knows is wrong and being really defiant. I'm trying to play with her when baby is sleeping and she's had one on one time with hubby and me. I feel bad because I'm constantly telling her to go in time out and we have even had to do some spankings. Next weekend my husband has to go on a trip and I will have both kids by myself for 4 days. Any ideas as to how I can get my sweet girl back. She has seriously been a turd.