need advice

hello! I need your help. right now I'm in a relationship (this is my first relationship and he has 25 years old and I have 23 years old), we've been together for 16 months. the first couple months he was the sweetest guy ever, he sent me at least one sweet message per day and he liked to talk to me any time, he didnt have excuse to see me or talk to me but lately he's distant, we don't talk like we used to talk, he prefer to spend time with his friends and before all of this I had weird feelings from him, like I feel him distant so I asked him if there was any problem and he said that there wasn't any problem but his actions doesn't transmit that. I don't know what to do, what should I do? I send him sweet messages, try to talk to him every time I can, I try to understand him when he wants to spend time with his friends, I don't push him to do something he doesn't want to do, I watch series he likes and play games he likes, but still feel that feeling. please help me, what should I do?