AITA for spending money my grandma gave me after I went behind my parents backs to get it?

So this Christmas my grandparents wanted to give my cousin and I an extra $100 on top of what we usually get. This money was supposed to be split between the 2 of us but my cousin wanted it badly so she stole it before we had a chance to get it. When I went to ask my grandpa for the money he had already given it to my cousin and she spent it so I went behind my parents backs and asked my grandparents for the same amount of money my cousin got that way I would have the $200 they were supposed to give us. They agreed to give me the money but they made me promise not to tell my parents. I felt like I had to do this because my cousin spent her Christmas money on an air fryer and a bunch of useless stuff while I had to use mine on a plane ticket for a college interview. My parents found out I had the money and asked me where I got it and I wouldn’t tell them til they told me I couldn’t leave the house til I told them the truth. I finally told them the truth and they got mad at my grandparents for giving me the money. They said it wasn’t theirs to give me and that I should have just told them I didn’t have the money they gave me instead of spending money they didn’t give me. I don’t see what I did wrong I’m just confused on why I’m in trouble.