Can antihistamines make my hypoglycemia worse? Seems like I'm having more low blood sugar occurrences while on Claritin (loratadine) or allegra.

Allegra (fexofenadine) I don't think so. If you really observe hypoglycemia when taking antihistamine then make you sure you eat 3 times a day, eat fruits between meal and just before your meal. Symptomatic hypoglycemia usually occur when blood sugar <80. We like to keep it above than 100. Symptom could include irritability, dizziness, sweating, weakness, confusion, passing out or death.
Probably not. There is no theoretical reason why antihistamines should lower blood sugar. It may be related to the reason you are on the antihistamines or some other medicine that you are taking or the fact that your dietary intake may be different at those times. Try to discuss this in detail with your doctor.