Why has her attitude towards you changed?

Really need more information to supply a specific answer. How has her attitude changed for the good? or bad? I'll assume it's bad simply because you asked the question. Some of the biggest reasons would be: she's found someone more interesting; your attitude towards her and/or ur relationship has eroded to the point she's negatively affected. If You care about this relationship U need to sit down an have a face to face ASAP