Hi, I've recently been preliminarily diagnosed with bipolar II (from my GP - awaiting psychiatric assessment) and have been put on Lamictal also.
I had been on SSRIs or SNRIs for years (antidepressants) and they made me worse.
I have bad trouble concentrating as you describe but I attribute this to depression.
I also severely doubt my diagnosis.
I wish I could tell you what it were like to be bipolar and help you decide but I have asked a very similar question here! (Perhaps it would be helpful to look at my question on my profile - it's rather long, sorry - to see if anything sounds familiar to you).
I would say that my depression has been severe enough to cause me severe distruption in life (uncharacteristically dangerous, violent behaviour, losing jobs etc,) but I have never seriously considered suicide so the fact that you have worries me. Are you still having suicidal thoughts?
It is possible you have unipolar (common garden, if you like) depression and not bipolar but I'm only really saying this because you haven't mentioned any "high" episodes above (which of course doesn't mean you don't have them) and, of course, depression is also a symptom of bipolar.
I would say that if your inability to concentrate is coupled with insomnia, high energy, feelings of elation, racing thoughts then you COULD be suffering a hypomanic episode (though it still could be anxiety). Whereas if you feel very low on energy, weak, achey, hypersomnia etc. your depression is causing the lack of focus.
These things are incredibly difficult to detangle to somebody not proffessionally trained to do so. I, too, think you were diagnosed rather hastily and I would have expected you to have more time with a psychiatrist for a full assessment to be completed. They'd need to know how you'd been, in detail, for years, surely. This is what worries me, also, as within 10 mins my GP diagnosed me with bipolar II based on a brief description of a feeling he identified as a "mixed episode" without really asking any questions at all.
I would recommend a second opinion and, if you are worried, ask to be referred long-term to a psychiatrist who can help to answer your questions.
Good luck with it, and good luck with those exams!