Aitah for swearing on her life and lying

Today my girlfriend (f20) and i (f19) were on the phone after a long day of not talking. We had both been busy and she was out with her coworker, and it was her first time really going out and hanging out with someone since she moved to another state. I month or two ago I was feeling a little insecure about this coworker, and kept wondering if she had a crush on my girlfriend. I had told my girlfriend about this irrational insecurity, and she said there had been questionable moments, but overall, there was definitely nothing there. I was gonna make a stupid joke about them thrifting together today, but stopped myself because I didn’t want to make my girlfriend feel bad or weird for going out with her new friend, especially since I knew there was nothing to worry about. She asked me what I was gonna say, and I said nothing I wasn’t gonna say anything. She asked me if I swear on her life, and I said yes hesitantly, only because I didn’t want to make it a big deal and make her wonder what the joke was. She said are you sure and I took it back and said maybe not on your life, to which she got upset about me lying about the swear. At first I thought maybe she was joking cuz she went quiet and repeatedly said I was being mean, but then she stopped speaking to me and was very obviously upset. I kept apologizing and explaining my reasoning behind my lie, which was to prevent hurting her feelings over the original stupid joke, but she is mad that I took that swear that generally meant we have to tell the truth, and lied. I know I didn’t mean to hurt her feelings, but am I an asshole for lying about it in the first place, regardless of intention and my immediate taking it back? She says she understands where I’m coming from but she doesn’t wanna swear on our lives anymore because it lost its meaning.