What you have is probably intertrigo which is an inflammation (rash) appearing in the body folds like groin, back of knee, neck, axilla/armpit etc. It can be red and raw-looking, and may itch, ooze, and be sore. It can be a bacterial, viral or fungal infection. Please keep the area clean, dry and apply some topical antiseptics. OTC anti allergics will help. You can apply topical creams with zinc oxide and mild steroids and see if it helps. In case the symptoms persist then antifungals can be applied (Clotrimazole 1%, Miconazole nitrate 2%, Tolnaftate 1% and Butenafine hydrochloride 1%). Many OTC products containing these salts are available, your pharmacist can help. Try a combination of two and apply until the infection you see disappears. Other than that it can be an allergic reaction to something. Sweat dermatitis too is a possibility. For confirmed diagnosis please consult a skin specialist. Take care!
Well, without a clinical evaluation it would be difficult to determine the cause of the lesions. Possibilities that may need to be considered include subcutaneous bruising, skin necrosis, initiation of cutaneous erythropoiesis, dermal invasion of cells etc. I would suggest getting this evaluated by a dermatologist at your earliest convenience for an accurate diagnosis. If no cause is apparent on clinical examination, a skin biopsy may be required to come to a conclusion. After the cause is identified, it can be managed accordingly.
Hope this is helpful.