2-3 weeks ago i had an ear infection and was given ten days worth of generic omnicef, (cefdinir) now i'm sneezing, coughing and have a throbbing headache?

Time for a recheck: Recurrence of an ear infection is common in the month after the germ is first cleared as fluid can often remain & become re-infected. It is also quite possible that another unrelated illness has come along.

See your doc...but..: Most likely this is another infection, like you picked up a viral infection on top of the ear infection. The ear infection may have been viral to begin with as well. With this being 2-3 weeks later, again this is likely another infection. With your symptoms however, i recommend you see your doctor now to assure there is nothing more serious going on.

Different problems: Sounds like two different issues. First sounds like a viral illness which you may or may not have required an antibiotic. The second could be allergy or migraine or another viral illness. I would see a specialist like an otolaryngologist or a good primary care doc that can point you in the right direction.