As the page Julien links to says, the batteries do have a shelf life. My experience is that you usually start to really notice you're getting less battery life out of smartphones, laptops, etc. at around the 2-year mark. It's a gradual decline, but I usually seem to notice it really become an issue somewhere between 1.5 years and 2 years.
My series 0 38mm sport hit the two year mark in July. I notice it doesn't hold a charge as well as it used to, but it's doing better than my iPhone 6, which is not much older.
The question is whether what you are experiencing is gradual wear and tear, or whether one or more apps are running amok and running your battery down. If I run a one hour workout, then my watch will often be down to 15-20% when I get home. Sometimes it's even at 10%. One year ago that was what I would expect from running a two-hour (or longer) workout.
I have had a number of occasions over the last two years when a sudden and dramatic drop in battery performance for a few days (or even up to a week) appeared to be mostly caused by a third party app. The exceptions were when I would have a few days of short battery life following a watchOS update. My guess there was that the update cleared some cache and forced the watch to re-sync a lot of data with iPhone apps over a day or two. The other battery life issues would go away after installing some third party app updates. If Apple only gave us a report on how much each watch app was consuming over the last 24 hours or 7 days, we could figure out which apps are the battery hogs. We have access to this information for the iPhone, so sometimes I've gone on the assumption that if an app has been using a lot of iPhone battery, and it has a watch app that is set as a complication or is docked, that watch app is probably using more battery as well.
In your case I wouldn't worry about it for a day or two and see if it was just an anomaly. If you continue to have a problem, check your app battery usage on the iPhone to see if you can identify any potential culprits. Try removing those watch apps until that app is updated by the vendor. It could also be the battery itself though, so another solution is to get a second watch charging cable for work, and charge it up mid-day. The watch can charge quite a bit in a very short period of time.
Sean
