My LO is 8 mo. 20 lbs. I gave her infant Tylenol (acetaminophen). She has a thick runny nose and watery eyes (allergies) can I also give her children's benadryl?

Benedryl indication. Although the manufacturer does not indicate it's use in children under 2 years of age, it is not uncommon for it to be recommended 'off label' by pediatricians. Oral cetrizine is another antihistamine for allergies which does have a documented age indication for 6 months and up. Both of these medications can be safely administered with Tylenol (acetaminophen).
Not recommended. Antihistamines such as Benadryl (diphenhydramine) can cause paradoxical agitation and are not routinely recommended for young infants especially for viral colds.