Do you have a thermometer? That would be be the best accurate way of telling the temperature :) For a 20L (5 gallon) you could get a heater, which then you don't have to worry with the "guessing game"
Know how I said, the more water, the less likely it is to hold heat? That could be why. The water is colder than it used to be, which can make bettas lethargic. There is also a chance that lower down in the water it is cold, while the top is the warmest there is. Is there a filter at all? If so, try removing the filter, and perhaps add a bubbler to keep the water circulating and that way the heat will also circulate.