My boyfriend's best friend

Okay so I've been dating my boyfriend (Matt) for almost a year now. I met him through my best friend's (Chrissy) older sister (Maria). Chrissy, Maria and I have been friends for about 20 years. Maria and Matt have been bestfriends for 6 years and were best friends long before I met Matt. Originally, I thought I would have no issues with their friendship because I've known Maria for so long and because I trust Matt completely. Lately though, I've begun to get jealous of their friendship especially when Matt talks about how much he misses Maria (they don't see each other often because of work schedules).