Is it Psoriasis?

I started with scalp problems this summer. At first I thought I had just been bitten by mosquitos on my head, but as they got flakier and crustier I went to see the doctor. She said it was seborrheic dermatitis, but I'm not convinced at all. It's not greasy, nor yellow. Just thick crusts of silvery white patches that are really itchy. She prescribed betacap, and it does clear up, but as soon as I stop, it bounces back within a few days. This week though, it has become weepy, to the extent that my hair was sticking together in places. Yuk! I am not sure what it is. Is it psoriasis, or seborrheic dermatitis? The reason I think it is psoriasis is because I have joint/tendon problems in several places around my body (feet, Achilles, hips, shoulders and elbows on and off). I randomly googled scalp and tendon pain, and bam, up pops psoriatic arthritis. Any advise greatly received. Thank you, xx.