Extra payment for already signed contract

I have been a member of Action Plus gym for the past 8 years or so. This year when I needed to renew my contract in July, I was a bit sceptical but nevertheless renewed it due to convenience. My new renewal contract was signed on the 20th July 2015 and became effective as off that date, with my first payment to go through on the 3rd August. When I was enquired as to whether I could commence my contract and start gyming on Saturday 1st August, I was told that because I came to gym on the 24th July, I had to pay for the entire month of July first before I could use the gym. I tried to explain to them that my contract was in effect from the 20th July and even had received a confirmation email from them to this effect. I was not allowed to utilise the gym, even though I had a contract, and am still not allowed to use the gym until I pay for the entire month of July. Thus I am left stranded and without a gym to use yet there is a signed contract in place.