Here are a couple of things to check off the list before you get too worried,
-any added stressor, (work, more workouts, family problems, etc)
If all of those things are in check, and you still feel low on energy, it might be worth getting a cbc, complete blood count to rule out anemia, thyroid issues, etc. Sometimes labs can be very telling.
Also, I just had some blood work done recently because I'm anemic and my vitamin d3 levels were a bit on the low side and I felt like shit all winter. The weather has been warmer and sunnier lately and as a result, I think D3 levels are higher so that could also play a role if you live in an area with long winters.