Okay, I say this. It doesn't make sense for your girlfriend to say you are everybody's chauffeur if this was a one time deal. It has happened more than one, I'm sure of it. Why am I sure of it? Because you admitted in your reply that you have a hard time saying no.
The next time someone texts you to something you need to let them know that you will talk it over with your girlfriend first. If your girlfriend disagrees and you really want to fulfill the request... ask you girlfriend to ride with you. That way you are including her and not disregarding her feedback.
For one, when you are in a relationship you shouldn't be doing other duties every time you are called upon. If they are your friends you should try talking to them about being independent!