Why doesn't she like him?
I would really recommend asking her. There must be something she feels is not right. This mustn't neccessarily be something you find problematic, but on the other hand she is more detached and might just see something you are not aware of yet.
Example: she could find that you are not paying attention to your feelings and wishes, neglecting your old friends for his sake and not doing the fun things you used to do to please yourself.
She could be seeing that he is manipulating you by putting on pressure, lets say by getting into a huff if things aren't going the way he wants...
Often Mums don't like to see the way the personality of their daughters change to please a man. In the best case this is not neccessary and often things start to balance after a while, but sometimes things get out of hand and a mother seeing "her girl" giving up her personality for a man, may well be opposed to this man.
She will want you to be happy and I guess she thinks that long term happiness is not going to come with this boyfriend.
On the other hand, it is you who has to get along with the boyfriend, not her. But it is difficult to see desaster lurking and to let your daughter walk into it.
Could it be something like this?