Fever with cough, runny nose and a bad headache is most likely acute viral upper respiratory infection, and 2-3 days of rest is genuinely justified. A sick leave certificate can only be issued after an actual examination - online or in person - not just on a message, so get seen first and the certificate follows the same visit.
Next Steps
Paracetamol 650 mg SOS, steam inhalation, warm fluids and rest. If fever persists beyond 3 days or you get breathlessness, chest pain or drowsiness, get examined the same day. Book a consult with me on Practo and I will assess you and issue the sick leave certificate if it is warranted.