Thanks for the advice on looking at the string buzz thread. That's exactly what it was. I read the instruction on the forum carefully and adjusted the truss rod step by step and it plays beautifully again.
Thanks!
P.S. I would've taken it to the shop where I bought it but it's not in a very convenient part of the city.. I would've had to take a bus and transfer twice to get there, I thought I'd try myself since there is a good isntruction on how to do it, and I killed two birds with one stone: I fixed the problem and I learnt how to make truss rod adjustment
