What's causing my tiredness, pollen allergies or the Claritin (loratadine) I take for it? I have been so tired since pollen season started.

Sleep. Sleep apnea could be the main reason but Claritin (loratadine) does have a mild sedative effect in some more than others. Sleep disruption because of allergy symptoms is also common. Allergy itself is not supposed to cause tiredness but I can't deny that many patients do have this problem.
Not clairitin. Claritin (loratadine) is supposed to be a non-sedating antihistamine and therefore should not be the cause of your fatigue. The allergic response to your allergens is the most likely cause of your fatigue. Consult your allergist.