I found this word a little strange. It can mean either depression or low spirits and can also mean a whim, fad, or a fancy. Or a species of left eyed flatfish. Or another word for migraine. Hmmmm.
Usually if a word has multiple meanings they are kind of similar. In this case, if someone says ‘I have a megrim’ you would just have to nod knowingly and wait for the next sentence to work out if they had a headache, felt sad, been shopping and bought a something unexpected or caught an ugly fish. Maybe it is really just another word for confused….