A burning, itchy, bumpy rash that came on over two days along your jaw sounds like an allergic contact reaction, possibly to a skincare product, soap, jewellery or something new you ate. Cetirizine once in the morning and night is a reasonable first step and should ease the itching.
Next Steps
Stop any new cream, soap or cosmetic you started recently, avoid scrubbing the area, and switch to a plain, fragrance-free moisturiser. If it hasn't settled in 3 to 4 days despite the cetirizine, please consult a dermatologist or physician in person.
Health Tips
Go in urgently if your lips, eyes or tongue swell, or you get breathing difficulty, since that needs immediate care.