What will happen if I have a Flonase overdose?

Overdose of flonase? I don't know how someone overdoses on Flonase unless you sprayed the entire bottle in your nose. You will very likely have a bloody nose, dryness of the nose and mouth, headache, irritation, possibly decreased sense of smell. Hey...Let's not try this and find out. Follow the directions of your doctor.
Depends. How big an overdose? If its just a few extra sprays a few times, likely nothing will happen. If it is many sprays a day over months or years, you run the low risk of systemic absorption and typical steroid side effects. This can include secondary infection, weight gain, cataracts, liver changes, osteoporosis etc. This would be rare and require a massive overdose.