The pimple that you have described is probably suggestive of a condition known as Folliculitis that is the inflammation of the hair follicles. It is commonly seen over the face, scalp, thigh and groin following friction between the clothing, excessive perspiration, and dermatitis. It is mainly caused by bacterial infections such as Staphylococcus aureus, although fungal infection also can lead to this condition. Folliculitis generally presents as a pimple with a hair follicle in the groin area associated with or without itching, redness and usually contains pus, blood or both. The pimple on your left leg may also be due to folliculitis.
I would suggest you to get a set of investigations done like complete blood test and use an antibiotic like Augmentin 625 mg twice a day after food for 5 days and a painkiller like ibuprofen twice a day after food for 5 days. You can also prevent further infection by taking regular showers using antibacterial soaps, use loose clothing, Avoid shaving the area for sometime and popping the pimple. If the condition gets worsened, then you can consult the dermatologist for further treatment.