Welcome to WHL, Federico. Psoriasis is AI, and there is an arthritis with psoriasis as the main "symptom" - psoriatic arthritis - surprise! A fellow I work with has it, and he has some bad rounds with it. It seems to knock his immune system down as it's attacking, much like it sounds like happens with you. However, that pulmonary embolism is not a minor symptom. If your lungs hadn't caught the clot, your brain would have... Not good. I can't remember for certain (I think that fog that Mari was in is thickening over here... tic), but they should have been able to tell from blood work what the clot was. They can't always tell where it came from, but perhaps you banged your shin on something a few days before? Some other impact to your body that could have damaged your blood vessel(s) and caused a blood clot? Just because you're "young", doesn't mean you don't have risk factors. Further investigating into that is definitely called for. Do you tire easily? Do you have difficulty thinking clearly when your skin breaks out? Do you have achey joints and/or muscles? Do you bruise easily? Start thinking of things like that, write it all down, and take the notes to the doctor. Take pictures of your skin when it breaks out, and show those to the doctor also. Best of luck with it, but be sure and check further into that pulmonary embolism. Those are dangerous.
"There but for the grace of God, go I."
"... His mercy endureth for ever."