All you can do is sit her down, and have a quiet, calm and honest conversation. That is if she wants to be honest with you. You aren't dumb, and you are noticing she isn't giving this as much effort anymore. Ask her how she feels and that you noticed the distance. Excuses like "I'm tired, busy, etc." do not cut it, and you let her know that they are reasons not excuses.
Okay, so she's not giving you a lap dance in the middle of public or sending you cute texts all the time. Doesn't mean she doesn't care anymore. However, if you are rarely seeing each other and don't even talk or communicate anymore, it's apparent you are growing apart. That's the whole point of a relationship is to grow together. She may have told you her feelings were as strong as ever, but her actions speak otherwise. Unfortunately, once something is broken, it's hard to fix it again if both aren't giving their full effort. She may have come running back the first time because she was scared of being alone, but it was only a temporary fix FOR HER and now she is gradually checking out of this relationship again it looks like.
Only she knows.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.