It is a double-edged sword, as far as I know, because if you have not had, having it would decrease your sexual desire for a while (at least it usually does). On the contrary, if you have had a sexual intercourse, it could help to have even more of it. I am not a medical doctor, but laically, people are experiencing such behaviour when they are sexually aroused or deeply in love (usually adolescent individuals). By a logical presumption, fulfilling or even overfilling the demand leads to a pause in the production process. Well, that has developed into an analogy, but it think that it is clear.
If this is not the problem, I advise you to take some time alone and analyse the situation and possible influencing factors. This is only general advice, but I know hardly anything about you so the best I can do is this and maybe telling you what works for me (not that I have a girlfriend) – well, I am interested in many fields of study and even if this may sound amusing, there are times when my mind is so deeply interested in one topic that I "forget" to eat, sleep and that stuff we all do. This obviously causes trouble because without sleep, there is not enough energy as well as in your case without concentration, there are no results. In cases like this, I find another interesting topic to distract me and then, when the distraction arrives, I suddenly look at the clock suggesting that it is about the time to leave it for a while. I do not know what your interests are, but try doing something enjoyable like playing Uncharted 2 and then move on to revising.
If you want any more advice, write something about your feelings towards him. Do you feel any pressure in your life, especially regarding time? Have you had a relationship that lasted for a short period of time? Do you feel that you need to spend lots of time with your boyfriend because of fear that you may lose him? Do you often imagine you together in situations that you have not experienced yet?
It is quite long, a real psychologist might be a better option.