I often get random patches full of itchy bumps on my body mostly on my forearms they're not bug bites or an allergic reaction. what causes this?

Many things!: What you are describing is a very common condition and not specific for any one diagnosis. You could have a contact dermatitis (an allergic reaction to something coming into contact with your skin), hives, eczema or any number of things. If your primary care physician is stumped, please consider seeing an allergist or a dermatologist.