One of the things that changes immediately when you're in a relationship or married is the experience of sharing friends. It happens so naturally that the notion of jealousy initially seems far fetched and completely unreasonable. However, the minute you begin to share your husband with your friends, it can spark jealous feelings for several reasons.
First, no matter how secure you may feel in your marriage, competition among friends is as old as the hills. Therefore, the minute you see your friend and your husband talking or spending time without you, it's not unusual to find yourself feeling either left out, threatened or both. This can happen simply because as long as you are in the equation you won't feel anything is being taken away from you. Once, however, you're not a part of it, it's easy to feel that the time your husband is spending with your friend is in fact time you want him to be focusing on you.
Second, rivalry over appearance can get triggered at the drop of a hat. If your friend is attractive and you're having an off day/night where you aren't feeling sexy and beautiful, suddenly instead of feeling proud of how fabulous your friend is, it can turn into...uh oh, do I have to worry my husband will find her prettier than me...
The antidote to all of this is to trust that your husband is genuinely interested in your friends because they are a reflection of you and that he's in love with you. Similarly, your friends are taking the time to know your husband since they care about you and he's married to you. That said, if you do feel uncomfortable or uneasy for any length of time then it's important to talk out your feelings with your husband so he can reassure you. And, if he is being a bit too flirty, rein it in. Finally, being your best YOU and feeling secure within yourself will go a long way to keeping jealousy in its place.