Me(f) and my fiance(m) got into an argument about a prank my bestfriend did to cheer me up because we worked around dangerous people

Hi I need advice about an argument I had a few years ago with my now ex. So at the time I was (F24) and at the time he was (M29). We had been dating for 2 to 3 years at that point and were going to get married. We lived together however around the time of this argument he started acting different. Now to the argument. So we were walking into the grocery store and he noticed a smiley face drawn on the side of the back of my neck he asked "whats that?" And I got confused because I and completely forgot it was there. I said "what?" And he pointed to it and said "the smiley face om your neck. Whats that about?".

After realizing what he was talking about i told him my best friend (M) drew it to make me cheer up.I was upset because earlier that morning me and my fiance got into a big fight (I dont remeber about what) but it made me really sad. Well at my job we couldn't be upset or moody because i worked around dangerous people. So my bestfriend who noticed I was upset tried to cheer me up. He played funny songs, sent me funny memes and reels, through paper clips and paper balls at me, he even tried funny stories and treats/food. Nothing was cheering me up. So finally my best friend decided to prank me. He waited till I got off the phone with a civilian. I thought he left the office but he had snuck up behind me. He then quickly drew a smiley face on the back of my neck ( I was wearing briads that went down my shoulders) I turned around and smacked him as he ran out the office into the front area where we let civilian come in to talk to us. He started laughing and honestly I started laughing too. It was stupid and childish and that made me laugh. We got back to work and I quickly forgot about it because to me it wasnt anything important.

Well back to me and my fiance. We got into this big fight and he accused me of cheating even after I explained what happened and I let him look through my phone completely. I told him that it was just a dumb prank and nothing important and thats why I forgot it was even there. My fiance dropped me off at home and disappeared for the night when he came back he showed me a reddit post he made and everyone was saying I was a monster and that he should leave me. One comment called me a W**** and said I deserved to be alone.

Anyway I keep thinking about it and I dont know if I was wrong. Also just so everyone knows my best friend is like 10 years older than me and hes married to my other bestfriend.

TLDR: fiance got angry about a prank me bestfriend played on me to cheer me up because we worked with dangerous people