Seasonal allergy, after taking antihistamine itch/sneezing got better but still has blocked nose, now jaw pain too?

Maxillary Pain ? Very common with allergies or sinusitis. Although not as common, I've seen the pain referred to the mandible, too. The problem should pass when the season ends.
2 separate problems. 1. Seasonal flu is very common viral infection with symptoms you have described. Usually the condition resolves on its own with home care. Drink water and get a good rest. NSAIDs (e.g. Aleve) are also recommended every 6 hours. 2. Jaw pain due to excessive teeth grinding and stress. See your dentist for examination. Take care.