What could cause low grade fuo's in my 18 month daughter? She has no other symptoms except low appetite.

Many things... Could be a simple viral illness. Teething could possibly cause a low grade temp and not wanting to eat. Bladder infections could cause some nausea, vomiting and temp. If by low grade, you mean 99 - and she's otherwise looking fine, acting well, even if not eating as much (toddlers are finicky), i'd give it a few days. If anything changes, see doc sooner.