Surprise! Charged for bag going home but not when I left

I actually booked my flight from Okinawa Japan to Tokyo and left Okinawa with no problem taking my small suitcase as a carryon. When I arrived at narita to go back to Okinawa the checkin terminal would not give me a boarding pass and told me to see an agent. The representative started talking about checking a bag, I told her it wasn't necessary I have the same bag I came from Okinawa and did not have to check it 2 days prior. She told me I had to weigh both my purse and carryon and they had to be less than 7kg together! No airline weighs a purse except these people. I will never fly with them again. It was a last minute trip I will stick with more reputable airlines.