This type of "lump" (for it's size as well) would not be very common of that of a pulled muscle/tendon, etc. in the area which you have described . This is more localized, as this may be more of a nodule, boil, subdermal infection, abscess, cyst, swollen lymph node, or others. Because you have had this for about a month, I wouldn't consider this to be an insect bite or a simple pimple.
I would suggest having your primary care physician take a look at this area when possible. He/she could examinate the area, in order to properly diagnose and treat if necessary. Because you are experiencing "extreme pain" at the affected area, (this is common of infection) I would consider this to be a possibility. There are other causes and conditions which may contribute to this, and be painful, but without further examination would be difficult to determine for sure.
If you experience any fever, chills, redness in or around the affected area "lump", or anything out of the ordinary, I would suggest that you go to the ER for evaluation and treatment as necessary.
If you have any further questions or concerns, please do not hesitate to ask.