If you develop itching and redness of the skin, it may indicate an allergic reaction to something you ingested. It could be from St. John's Wort or something else.
I would recommend stopping the St. John's Wort for a while.
You can also buy an antihistamine from the pharmacy to help control the itching.
If the problem persists, you should definetly seek help from a doctor!
PS: St. John's Wort can cause increased sensitivity to sunlight, especially after long-term use. Always consider that herbal remedies can have side-effects. As each of us is different, you never know how your body will react to the treatment.