Sometimes you can start to sense something is wrong in a relationship when it has nothing to do with the relationship. For example, the other person could be having problems with family or work or school and is distracted from your relationship. The distraction could cause them to act differently.

Can you ask them if things are ok between the two of you? It takes a lot to ask because there is that fear it could be over, but you might find out for sure where the 'distance' is coming from. I hope they are not wanting it to end and are preoccupied with something else.