How am I supposed to feel?

It's freshman year! about two months in and two days before my birthday. The guy I have liked since the start of the year or shortly after was seen making out with some girl. I'm numb. how am I supposed to feel about that? He said it was just a hookup type thing and meant nothing.. I'm glad though that I stayed home from school with a sick feeling. I can't seem to put a facial expression on my face. I wish I would cry... or do something, but I am lost. What does anything mean? Was I so naive to fall for someone like that? I would hope not, but I'm so confused. What now? How do I respond to this? I heard this through a trusted source and I bet he doesn't know that I know... How do I act?