What are the first few signs of an allergy?

[?StarBall]

The other day I got a rash at night at the back of my neck to my shoulder. (Day 1)

In the morning when I got up it had spread to the front of my neck but after a while it subsided. And then I got a slight headache (Day 2)

And the day after that I had a fever and my stomach felt as if I had gastric but I didn't. (Day 3)

Same for the day after that again, which was yesterday. (Day 4)

But now I'm ok, except the rash is still there but not as bad. So is that an allergy? I have no idea what I ate though... it's not like I'm sick or anything now, just the rash.
I haven't eaten anything new, but a week before that happened I changed shampoo, conditioner and soap o_o