Dr. Says I have a foreign body under my conjunctiva. Could it be anything else that can cause similar symptoms?

Unusual. Were you struck in the eye with a fast moving object.. Most objects striking the conjunctiva will embed in his tissue, and if they penetrate, will set up an inflammation. If it was fast enough to penetrate, there is a risk that it might have penetrated the eye. Is you vision OK? You might see an ophthalmologist to have this looked at and straightened out.
Unusual. Definitive diagnosis can be made with a slit lamp examination by an ophthalmologist.
Follw your doctor. I suppose by doctor you mean opthalmologist(eye specialist) if so follow his/her advice otherwise see an eye doctor for second opinion.