At the end of a rotation, it is acceptable to ask a faculty member who gave you a good evaluation to write you a letter of recommendation.
Some students prefer to wait until the third year is over before requesting letters. This is also acceptable, but asking soon after a clerkship ends does offer you one advantage. If the writer drafts the letter after the clerkship, you are fresh in the writer's mind. A letter written at this point in time is more likely to include specific details and examples that back up any praise. Letters of this sort are more effective in supporting your application. During the residency application process, you can ask the writer to update the letter incorporating information about your specialty choice and career goals.
When asking, be sure to ask the attending if he or she can write you a strong letter of recommendation. If the attending says "yes", then you can be confident that the letter will be supportive. If the answer is "no", then you can move on to the next attending on your list of potential letter writers. Do the same for any attending who seems hesitant or unenthusiastic about writing the letter.
I understand that you may feel awkward about asking but remember that attendings expect to be asked to write these letters for students who performed well. It's just one of their many professional responsibilities. Too often, awkardness and hesitancy get in the way and students procrastinate. Don't procrastinate when asking for letters because if you delay, attendings may not be able to write the best possible letter because of time constraints.
Please feel free to contact me if you have any other questions.
Good luck securing those strong LORs,
Samir Desai, MD
Assistant Professor of Medicine
Baylor College of Medicine