Please give me some advise about my skin allergy

Recently I found that my skin become very sensitive at night (just after 8pm). I have rashes that looks like mosquito bites all over my arms and legs, besides that can see some red dots and patches beneath my skin. The rashes are itchy. But usually they will gone the next morning. Sometimes my lips swollen for no reason too. I did see doctor but seems he can't help me too, he said its some sort of allergy, what I can do is just avoid the allergen (which havn't been found). He gave me piriton and calamine lotion, which only heal the symptom but not the cure. So every night I'm struggling between endure the itchiness and work or take the pills and sleep, it's really a hard time for me.

Actually few years ago I did gone through a bad diet, I took slimming tea (which make bowel move) almost everyday for around one year, I stop taking this tea when I found it make my body weakening. Is that a reason for my sensitive skin? What can I do now?

Some of my friends suggest me to take vitamin C and vitamin E. Will that help?