What are the advancement opportunities like? Myabe you're subconsiously looking ahead and feel "stuck" where you are. Do you see yourself with this company for the next 5+ years? Maybe you just don't feel challenged anymore.
Last year I did the same thing you did; went on a week long vacation to try and re-energize myself. But when I got back to work I quickly fell into the same malaise I had before. It became even more clear to me after I got back from vacation that this wasn't what I wanted to do, and I ended up quitting the job 3 weeks later. I've recently been offered my childhood dream job, so it all worked out for the best.
At any rate, I hope you find what it was that got you excited about the job when you first started. But always keep an eye out for better opportunities too.
Cheers!