Fedhealth bad practise

I cancelled my cover with Fedhealth due to unaffordability. They then decided to go back to previous months and reversed numerous claims(well over 30 days) which i had paid them and was covered for. Where do these providers get off. you pay them a huge amount of money everything month and they still try and shaft you at every turn. You would think they are one of the best but honestly they are all the same.