My bicycle didn't produce any squeaks until the last weekend, when I decided to make a little maintenance. First, I used "Easy Clean" spray from the White Lightning brand to clean the cogset and the chain. Then, I applied "Epic Ride" semi-dry all conditions silicone lube from the same brand to lubricate the cogset and the chain.
When I rode my bicycle afterwards, I noticed squeaking sounds coming from the cogset. Why might this happen? Did I miss something in my maintenance procedure? Did I use wrong type of lubricant?
I've found some useful information from Why is my chain so noisy after cleaning and lubricating? At this point I believe that the squeaking sounds are caused by firm particles (sand) that got into the bearings or onto moveable parts in the process of degreasing. In the beginning, my suspicion was that I had used wrong type of lubricant (why they put silicone in there, anyway?). I don't think I can select the best answer from those provided below (can't upvote them either because of the low reputation), but I appreciate the effort of those who shared with me their thoughts.