I heard the word 'selfish' mentioned by mom, brother and grandma a lot. And, to me that says you used to be very much a doormat, doing everything for everyone else (except yourself) and they got used to the freebies you were offering and got used to be coddled and helped out. And, maybe they took you for granted. And, now that you are grown up and doing your own thing, you are seen as selfish cause you won't let others walk over you and you won't run errands like before.
Maybe your mom thinks that's selfish cause it means more burden on her. Look at it from her point of view. If you're not at the house a lot, then that means you don't do your fair share of housework, so more work for mom. Of course, she is going to think ill of that. But, she should not call you selfish, cause you have your own life.
Maybe you had more time before to hang out with your brother when both of you were younger, but with school work and job, you aren't able to do so anymore.
You need to sit down with everyone and explain what's going on and how you wish you could be there more but have new responsibilities that take up your time. Explain that you're not going to be able to run around doing errands for everything like you did before. Tell them the mean words hurt. Try to find a fix for this situation before it festers and splits up the family. There is more than one side to this situation. If people are saying you are selfish, then pin them down on the subject and make them explain HOW you are selfish, then either dismiss it or work toward a solution.