Is this possible: already opened jar re-popped when opening?

I bought a jar of pesto a few months back. It's half used and has been sitting in the back of the fridge. I was going to use it tonight, but when I tried to open it, I had to put some force into it and it gave what seemed to me to be a "pop" as it opened. Looks and smells OK... there is a layer of solidified fat on top. Thoughts on safety? Does repopping mean bacteria has built up inside, or is that not possible and I'm just paranoid?

FYI I opened it in the fridge, so I don't think it's from temperature changes and air expansion.