No, it isn't too early. The muskrat would have stayed there over the winter. They are nocturnal, so they're mainly active at night. (But it isn't uncommon to see them out during the day). That may be one reason you haven't seen it. Also, young are born starting in April, so if it was a female it may be tending to young and not out as much. The other possibility is that it did not survive the winter. Muskrats do not live very long. Flooding is a major source of mortality.