I don't know why people think i'am conceited?

maybe they feel neglected by you because you spend time with him (how much time I don't know but if it a lot more than with them that may be the reason they are being so harsh). They might (though I'm not sure) be bit jealous if they don't have a boyfriend and feel they need to put you down to feel better about themselves. If it's either of the two maybe just try to put aside more time for them and show them how much you value them as friends.
However maybe you might be coming off as concieted without realising. Do you boast a lot about yourself and don't show much interest in them? Make sure when you are having a conversation that it's not you just talking the whole time.
Another reason may be that they just aren't accepting of the relationship and of you. Ignoring someone is pretty harsh and not the right way to handle conflict.
I think you should just talk with them to figure out really what is the problem.

Maybe they are just jealous because you have a boyfriend. If they have a boyfriend too then maybe they are just jealous because your boyfriend might be sexier than theirs or kinder or a better personality. Try to spend the same amount of time with your boyfriend and friends so none of them feel left out or anything like that. Or if that is not it then maybe you just need to talk with your friends and ask them what is going on. Hope that helped please vote up and/or leave a comment! :)