I feel like my freinds are taking advange of me what do you think i should do?

This has happened alot really my old best mate has paried off with my flatmate they go everywhere together, do everything together and alot of the time i feel taken advanage of and left behind. We all planned to have an 80's film day in watching t.v one decided to get food another invitee some snacks me and my flat mate were going to get drinks (i.e bottles of coke) together cos we don't have cars it means we would have to carry them across town if their were weren't 2 of us it would be almost impossible to do. But my flat mate decided she is going to bake some brownies instead leaving me to do it all on my own i'm aching from my martial arts cl's i'm really small a size 8 and 5ft2 and they expect me to just lug back liters of coke on my own i can't do it when i said i needed help they told me i was being selfish and to just get on with it. Now i feel like i should just not go and get the bus home for the weekend.