It may be that your guilt explains the first night on his birthday. You may have been feelings guilty and then felt insecure because you weren't there with him when you feel you should have been. Since that time your insecurities are coming through. If you don't think they did anything they shouldn't have then this is about your insecurities. Your description of her really suggests that you are more worried about her physical appeal to your husband rather than real evidence that anything is happening. Insecurity can cause some serious concerns that aren't valid. Your description of her proves that you are for some reason insecure about her and less secure about you. You can't let your worries about her looks and her situation to overcome your security in this relationship. If that happens then it becomes jealousy and eventually the end of the relationship.
You have to talk about your partner about your insecurities. Ask for reassurance. Ask for him to be a loving partner. Ask to go along and show you in public that you are his choice. Involve him in resolving these feelings you are having. Then you will learn to dissolve the fears that you have
If this is helpful press accept