I think you have an upper respiratory tract
infection related to viral infection which so commonly causes headache as well. Your physician gave the antibiotics to prevent added bacterial infection or may be he thought of bacterial infection per se.
Anyway I think the diarrhoea
is related to Cefdinir.
Stop Cefdinir, have paracetamol+chlorpheniramine+pseudoephedrine
based decongestant tablets almost 3hourly for three doses then four times a day for 3-4 days.
Maintain a good oral hygiene
and hydration and do saline gargles thrice a day post meals.
Drain the sinuses- lying in bed on side postures with blocked nostril
Get base line investigations through your physician.
I hope the advise would be helpful for you.