Unlocked phone is re-locked?

I had my samsung s4 mini unlocked a year ago and traveled outside the US where I bought a pre-paid plan/sim card. I put my AT&T sim card back in when I returned. I'm traveling again and just spoke to an AT&T customer service rep who looked at my account and said my phone is locked. Does AT&T re-lock phones? I went through a major hassle and problems to get it unlocked last year. Don't want a repeat!