Asking this again... revised... answer please?

okay so today me and my boyfriend both had free period together. hes 18 and im 17. we went outside and we were just laying in the gr when he started kissing me then he got on top of me and continued to kiss me. he put his hands in my shirt( nott on my just on my stomach) i put my hand on his belt and he immediatley pulled away really suddenly. ive done stuff with him before so i dont know why he did this. i asked him later and he just changed the topic really quickly. weve been dating for about 4 months i think. why did he do this i wasnt even going to do anything i was just puttin my hand there.