Allergies or teething or ???

About 3 months ago my boy was prescribed Cetirizine (Zyrtec) for seasonal allergies. After the appointment he was doing great! No obvious signs of difficulties. I took it as Spring allergies.. all the sudden he gets 4 teeth in and it's been drool city. Now he has the "cold" like symptoms.. really stuffy runny nose and a warm forehead with no fever.

Now I'm not sure if it's for sure the teething or if his allergies are acting up? So I'm not sure if I should try his allergy meds??? I'm a FTM, he's 20 months old and I have never had to deal with allergies so I'm not 100% what to go off of.

I feel silly for asking but can anyone give me a quick 101 on allergies and how to recognize a flare up??

Teething I know all about but the drool/cold symptoms have never been this bad.. thanks lady.