Playing offline made me lose items

Played Borderlands 2 offline for a couple of days, and when I went back online, I discovered that all my unlocked skins and heads have disappeared, along with everything in Claptrap’s secret stash. The items in the stash, where the ones I had the last time I played online. How can I stop that from happening? There’s no invisible feature on PS4, and sometimes I just want to play alone, so playing offline is the only option.