What to do

I love my boyfriend very much and he tells me he loves me. We've been dating for about 6 months and decided to have sex for the first time last month. Things just feel different and I don't feel like he cares for me the same way anymore. I don't really know how to bring this up to him exactly. I can't seem to pinpoint this issue or even remember when I first started to feel this way. I don't want to sound crazy or paranoid, he just doesn't put me first like he used to. I feel like we're just using each other for sex and I want to get back the relationship part of things that we used to have. Anyone's advice is welcomed; pls helppp!