It certainly is possible that this is a pimple, since those can occur anywhere on the body. However, the fact that the sore is present at the corner of your mouth suggest an alternative possibility to me, which is that this is a cold sore.
Cold sores are sores that typically occur at the corners of the lips. They tend to be larger and more painful than pimples. They are caused by a reactivation of the herpes virus, which most of us have been infected with at one point or another in our lives. In times of fatigue, stress, or illness
, the virus can reactivate to cause these sores.
Cold sores need to be protected, because they do contain virus, so you could theoretically pass the virus to another person through kissing and similar activities. Furthermore, although they do tend to heal up on their own, there definitely are medications that can speed up the process of healing and help to relieve the discomfort.
Therefore, I would suggest that you see your primary care doctor
about the issue. If this is confirmed to be a cold sore, then they will be able to help you decide whether or not taking a medication is a good idea.