I also think it is normal to go through periods where maybe you are not just that into your job (or in your case volunteering). Sometimes this happens to me, for example, after a project that required lots of hours with a hard and fast deadline. It can sometimes knock the wind out of your sails and it takes a bit to really get back into the swing of this with a great attitude.
However, I think once you start back into the routine things will fall into place again. If they don't maybe you need to evaluate what you are doing and if is the right fit.