Does anyone have any idea what is causing this rash?

for the past month or so, i've been getting sudden rashes that look like welts or big mosquito bites that seem to go away within minutes. i feel itchy & when i scratch, they turn into big welts & then disappear. they first started behind my knees, inner thighs & hip area. they are now down my legs, arms, wrists, back, etc. they are not bright red but more pinkish. i am a 39 year old female & never had these symptoms before until recently. anyone have any idea what this might be? really trying to avoid having to see a doc unless necessary