Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for your cat. I'm not surprised that you can't find a stinger since they rarely are left behind in dogs and cats; at least I've never been able to find one.
Unless her leg or where she may have been stung is swollen or appears irritated, you don't really have to do anything. However, if the area becomes a little swollen, then over the counter (at least here in the States) Diphenhydramine can be given at a dose of 1 mg per pound of body weight up to 2-3 times a day. For the average 10 lb cat, this calculates to be about 1/2 of a 25 mg tablet.
Other antihistamines may work as well; if you provide me with their names, I can provide the dosage amount.
I hope this helps. Deb
It might. The dose would be 5 mg once or twice a day.