Sorry you aren't feeling well. Well, if you think about all the spleen does, it would make sense that it is active during any type of response where white blood cells are being made. Infections are more common. I didn't find anything directly related to flu but more things like mono or true infections. The enlarged spleen means it is doing it's job and has worked hard. If it doesn't go down in size, then definitely schedule an appointment with your doctor to diagnose why it's enlarged. Let us know how you are doing.
Not a medical professional, but enlarged spleen is very common in mono. Might want to consider the possibility.
Source: currently have mono, have an enlarged spleen