On my calf muscle i had a rash .over night there's now slightly bigger bumps on it ..looks like a spider bite?

Allergic dermatitis: Allergic dermatitis can cause a rash composed of tiny red bumps over a reddened base. Occasionally an insect bite can cause localized swelling and allergic reaction. The best course of action would be to take an antihistamine to reduce the swelling/rash. Watch the area to ensure that the redness is not expanding outward which could mean a skin infection.