It is unusual for hayfever to make you feel light headed. However, it is possible - what happens is that any inflammation of the upper nasal passages resulting from the allergy will increase mucus production and swelling of tissues.
This can then impeded the natural drainage channels from the ears and may result in dizziness or light headedness. If indeed this is the case, we would treat the underlying condition of hayfever in the first instance and would recommend the use of A.Vogel Allergy relief
As you can imagine, there could be other causes of lightheadedness and it may be worth having your blood pressure checked, especially if you are on prescribed medication. If symptoms persist, do consult a doctor.