Henna and itchy scalp -- help!

I'm having some issues with henna and am hoping someone there might be able to offer some advice. I've hennaed my hair twice. The first time I used a mix of lemon juice, art-quality henna (from mehandi.com), a couple drops of tea tree oil, and a bit of paprika.  Now, my hair was dark brown so I had lightened it with bleach before doing the henna. The day after doing the henna I noticed that the glands on the back of my head were swollen and painful, but I figured this was from the bleach, so I didn't worry seeing as how I wasn't going to bleach before doing henna ever again. My scalp got intensely itchy shortly after hennaing the first time, and I thought maybe it was the lemon juice paired with the bleaching -- I attributed most of this itching to the bleach, so I didn't think hennaing again would be a bad idea. The itching has continued on and off for about a month. I hennaed again the other day and the itching is INTENSE this time around. I did a patch test which was totally fine, so I assumed that I'm not allergic to the henna. This time I used only henna and chamomile tea, hoping that I wouldn't get any irritation. The glands on the back of my head seem slightly swollen, but they aren't painful this time.  What gives? I've tried oiling my scalp with olive oil, I've done a diluted apple cider vinegar rinse, I've mixed a couple drops of tea tree oil with warm water and rinsed with that...nothing seems to work aside from putting hydrocortisone cream on my scalp. Could I be allergic to henna even though I keep getting negative patch tests? Should I avoid using henna for good if it causes this kind of itching? Other than the glands and the itching I haven't had any other allergy-like symptoms...no wheezing or tightness of the chest. My scalp is white and healthy-looking, no flakes or reddness, just intense itching!

 Its possible I suppose, since there's way more exposure with a full head henna. You really need to talk to your doc and give him this whole history.