Singing is a pretty intense activity when you compare to speaking, usually you sustain sounds much longer, use a very wide pitch range, a lot of energy, and the demand for quality is higher.
This also means muscles will work harder and, if you are not used to the effort, and specially when you are not used to and the coordination is still not quite developed, you can get a bit sore. Try to not overdo when you feel tired and you should be fine.
A great example of a similar thing is how our pinky hurts when first getting a hang to play the guitar and doing exercises that uses it.