Anyone else get allergic reactions to herbal tea?

Quick heads-up:
I tried this tea blend made of equal parts St. John's Wort, plantain, and dandelion root. Next thing I know, I've got huge hives popping up all over my head and my whole body is itching like crazy. I’ve been using St. John's Wort infused oil for massages for a while now to deal with my back pain and it works great, so I have no clue why one single cup of this tea sent me into an allergic reaction that's lasted all week.

Where did you pick up that tea blend? I’m wondering if someone just whipped it up themselves or if it was bought pre-packaged from a local health food store like Whole Foods.
One really ought to exercise some caution when it comes to these herbal mixtures—after all, you never truly know what might trigger an allergic reaction, and I suppose whoever is responsible for the "mixing" should be paying much closer attention to the ratios involved.

If I may offer a suggestion, perhaps it would be wise to see a physician; it probably wouldn't hurt to ask for a referral to a dermatologist for some allergy testing. That way, you'll actually know exactly what to avoid in the future.