I had this!! I woke up nearly every morning and it covered my arms, legs, back it was so itchy. My lips and eyes swelled up too, I went to my doctor and he thought it was due to allergies. I had tests done and they all came back negative... He then put it down to stress.
Looking back I was stressed: it was my first time flying on my own, I had exams and I was on my own for 2 weeks. But the problem was it didn't go away after the stress...
If it comes back again, go to the doctor, take some antihistamines or what also worked for me was drinking those bacteria drinks like yakult, my doctor said that it might be something to do with not having enough 'good' bacteria. in my body. After about a month I didn't need to take the drinks anymore.
Hope I helped and didn't complete confuse you with my post lol xx