Fatigue is what I felt when well after a long hike, dancing all night or being up several times a night feeding a baby or caring for a sick child. Everyone experiences fatigue. Such fatigue doesn't prevent doing it all over again the next day and the next day ....
ME/CFS fatigue is different, there's the very rapid fatiguablity when attempting activities of daily living, that includes significant reduction in ability to keep functioning, and ramping up of physical pain and/or brain fogginess, slow thinking.
Malaise is feeling ill. For me, PEM includes headache, nausea, loss of appetite, shakiness and dizziness when I try to stand, fainting, severe sensory sensitivities and inability to think straight, eg to follow a simple audiobook. Basically it means staying in bed in the dark, trying to get enough fluids without vomiting, ...
PEF and PEM are totally different.