It means that it happened very recently, perhaps only a minute or two ago, and I am probably still feeling the effects of the attack. The word 'just' is used with the present perfect tense to say that an action was completed now or in the very recent past. 'I have just had an attack'

here "had" means "got, obtained"

Please give the whole sentence, otherwise it's not really possible to tell what it means. Most likely it means 'in the immediate past.