What Causes An Itchy Red Lump On The Body?

There are no signs of bed bugs, fleas, mosquitos or knats, but I ll be sitting at work and just feel an irritation at a point on my body, so when I check, a raised red bump starts forming. Its itchy so I start scratching until it swells a little, but not big like a mosquito bite. I ve never had allergies before and was diagnosed with an allergy to dust as an adult, but never suffered this kind of reaction till this summer. I never had food allergies or allergies to medicine. I also have been suffering from an increase in phlegm in my throat that I end up choking until I can get a chance to loosen it and cough it out.