Costochondritis: This condition is characterized by inflammation of the cartilage that connects the ribs to the breastbone, which can cause sharp pains in the chest and back, especially with deep breathing. The patient's tenderness to palpation along the left costochondral junction supports this diagnosis. The recent physical activity (swimming) could have exacerbated the condition.

Musculoskeletal Strain: Given the patient's recent increase in physical activity (swimming), a musculoskeletal strain, particularly affecting the paraspinal muscles or the muscles between the ribs, is a plausible explanation for his pain. The description of the pain as "feels like ants" and spasming, along with tenderness in the left paraspinal muscles, supports this diagnosis.
Acute Coronary Syndrome (ACS): Although the patient's presentation does not strongly suggest ACS (e.g., no radiation of pain to the arm, jaw, or neck, and no electrocardiogram findings are mentioned), it cannot be ruled out without further investigation, especially in a diabetic patient who is at higher risk for cardiovascular disease.

Pulmonary Embolism (PE): Despite the clear lungs and normal breath sounds, PE is a critical diagnosis that must be considered, especially if there are any risk factors (e.g., recent immobility, family history of clotting disorders). The pain worsening with deep breaths could be indicative of PE, although the lack of other typical symptoms (e.g., sudden onset, tachypnea) makes it less likely.
Aortic Dissection: This is a life-threatening condition that requires immediate diagnosis and treatment. The patient's hypertension and the nature of the pain (constant, tight) could suggest aortic dissection, although the absence of other classic symptoms (e.g., radiation of pain to the back, unequal blood pressures in the arms) and signs (e.g., a new murmur) makes it less probable.
Pneumonia or Pleurisy: Although the lungs are clear, atypical presentations of pneumonia or pleurisy could still be considered, especially if the patient has been exposed to anyone with a respiratory infection or has other risk factors.

Herpes Zoster (Shingles): The description of the pain as "feels like ants" could suggest a neuropathic origin, such as shingles, which can cause severe, burning pain before the rash appears. However, the absence of a rash and the specific location of the pain make this less likely.
Tietze's Syndrome: This is a rare condition characterized by inflammation of the cartilages and surrounding tissues of the ribs, similar to costochondritis but can involve any area of the chest wall. It's considered rare and might be a diagnosis of exclusion.