BraveRifles

Usually with symptomatic (entire body rather than the local area) reactions the onset will be within an hour if not minutes or seconds depending on your allergy and the number of stings. Sometimes you may get hives, usually forming on the torso first and spreading to the limbs, or just redness. More severe reactions will include shortness of breath with wheezing, loss of conciousness, swollen airway to the point of obstruction, and even cardiac arrest.
The best thing to do if you suspect you are allergic after a insect sting is to take some liquid form of diphenhydramine which is the active medication in Benadryl. Take 50mg immediately for an adult and 25mg for children. Another way to do it is 1mg/kg of body weight to a maximum of 50mg. This is best for small children. If all you get is hives they should go away with the medication, if a more severe reaction takes place such as shortness of breath call 911 or get to an ER. Certainly call 911 if you are not near an ER because seconds count and they can give you epinephrine to open the airway back up and constrict the blood vessels to avoid loss of conciousness or any shock resulting from low blood pressure.
Some local swelling when bitten by an insect is pretty common and harmless, it is to be expected. Elderly persons feeling faint can be so many things on any given day, unless the actual bite is looking worse I would not worry too much personally, but I would be a fool to tell you not to consult her physician
Let me add don't take Benadryl lightly, if you call 911 with a severe reaction the paramedics will give you an injection of it along with epinephrine. It is a very strong drug that can certainly help your severe reaction, if you are driving to the ER take it first!