I am having a difficult time making a fist. What could this be due to?

My job is that of a dockworker. I drive a forklift and do a lot of heavy lifting. I've been doing it for a few months. Just this past week, I've noticed that I've been having a very hard time clenching my fist or gripping things in my left hand. My muscles are sore and my joints pop, but regardless of the pain, I just can't seem to make a fist unless I work it a little, and I can't press with the amount of pressure that I normally can to hold and carry things. Any ideas of what this could be?