If i start a party on my xbox 360 are the other players using my internet allowance.?

just today i checked my broadband usuage and was shocked to find out that i had used 49.77GiB and more than 40GiB was used on wednesday the 18th may i could'nt work it out until i sat down and thought about what i had done on that wednesday the only thing i had done was play on my xbox with 4 other guys in the usa in a party online playing team deathmatch on black ops,does anyone know were they using my internet allowance,i left the party but did that mean they were still piggy backing on my internet allowance etc,PS i'am with talktalk and i get a 40GiB a month download allowance and i never go over and i download movies,songs,browse the net go on utube and play online on my xbox 360,if anyone knows can u leave a reply thanks.pss also can anyone work out if it was 'nt my xbox were did 40gib go in 1 day was i hacked etc do u think,i asked talktalk to check but they said they could'nt because that would be snopping lol etc.