@shauna14,
Being jealous isn't stupid at all! I think you'll find a little jealousy in a relationship is slightly attractive. If a guy came up to me, interested, and my boyfriend didn't show concern, I'd feel a little unimportant.
What you don't want to do is be overly jealous. The stronger sense of trust you and your boyfriend have, the less jealous you should be. You should trust him to love you, and only you, and to be there for you. If you're afraid he's not going to stay, you may have an insecurity that you need to find and reassure yourself on. If you haven't been with him very long, just give yourself time. The trust will grow and the jealousy may die down on its own.