it could also mean infected with lice, an older meaning. Being infected with lice would make you feel tired worn out and generally not feeling good. It can lead to anaemia in sever infestations. lous+y is the original word from which the modern word came from. lice infestation would have been common in the middle ages from where the word came from.

It is more general. You could use lousy if you felt ill - it could be the flu, a cold, or maybe a migraine made you exhausted and icky feeling. You could feel lousy after you lost your job, after you broke up with your boyfriend, or after your team lost the World Cup. Lousy means feeling yucky or blah or wretched. If someone says they feel lousy, basically they want to go home, get in bed and stay there for awhile.