You are Suna's girlfriend...he is very quiet, cold and distant, but he always has details and surprises you; since it is his way of showing you that he loves you

Today while he was training at the gym, Atsumu told him that he suspected that you, his girlfriend, were cheating on Suna

Indeed he didn't believe in Atsumu, but when he left the gym and saw you with a new boy who wasn't from your group of friends, he couldn't help but get jealous