It sounds like your cat is healthy and cared for, so it's most likely not feline leukemia. It could very likely have something to do with the snow and winter. Seasonal Affective Disorder, or SAD, is not just common in humans but is also prominent in cats. Factors such as light, temperature, sleep, eating habits, and exercise could all be contributing things to the state of your cat. Based on the description and the fact that your cat started to behave this way after it started snowing, i think that is the problem. No need to worry about it though, it will pass! Here is a link on SAD in cats for more information:
Hope that helps!!