Hello. We Have A 6 Year Old Great Dane, Female,

Hello. We have a 6 year old Great Dane, female, fixed. Three days ago she became very lethargic and I noticed a mass on the left side of her throat. The lump does not seem to be painful to her and there is no puncture or bite wound that we can see. She just barely moves, has lost weight, eats very little, does drink water and urinates. She is almost non responsive to us when we talk to her. What do you think is wrong with her?