This sounds like it really bothers you that your best friend and your boyfriend are hanging out together a lot without you there. It seems from your question that they leave you out of the mix a lot and this hurts you. It also makes you feel a bit jealous. These are all quite natural feelings that you are having and TeenHealthFX would be a little curious too about the situation.
Sometimes the best advice you can get comes from within. You stated yourself that you know something may be going on and that you are uncomfortable with the situation as it is. It may be a good idea for you to talk to both your best friend and your boyfriend individually about your feelings and what exactly it is they are doing that makes you uncomfortable and jealous.
If they do not respond to your needs, you may want to move on from this relationship with your boyfriend. Examine the relationship and decide if you can deal with your feelings and still have a healthy relationship. TeenHealthFX knows that this is easier said than done, but in time you will make the right decision for you.
When examining and looking at your relationship, keep in mind that it should not be based around jealousy, feelings of frustration and loneliness, or unhappiness. Healthy relationships can be fun, but they are also hard work for both the people involved. The ultimate goal is that it be fun and fulfilling.
Sometimes this relationship stuff can be hard to deal with alone. Since your best friend is so involved you may want to talk to another trusted friend about it or even a parent, teacher or a guidance counselor about how you are feeling.