What Do Recurring Itchy Lumps On Scalp, Shoulder And Legs Suggest?

I get itchy lumps from time to time, similar to large insect bites. These often, but not always, seem to occur after i have been outdoors, for example doing yard work. But at other times I can see no pattern related to my activities. Right now I have several in my scalp, near the lower part of the back of my scalp. These return from time to time--every few days or weeks. I also have one on my shoulder and another on my leg. Bites? Allergys (I take allergy meds)? Something else?