Heat-related illness: Given the symptoms of chest tightness, shortness of breath (SOB) when in the heat, nausea, vomiting, and feeling dehydrated, along with an elevated heart rate (HR 120), this diagnosis is highly plausible. The normal BUN/Cr and the absence of significant electrolyte imbalances also support this, as severe dehydration or electrolyte disturbances are common in heat-related illnesses.

Viral or atypical pneumonia: The presence of cough, elevated WBC (9.5) with a lymphocytic predominance (lymphocytes 55.5%), and mildly elevated liver enzymes (AST 44, ALT 69) could suggest a viral or atypical infection. However, the lack of fever and the normal chest exam (not mentioned but implied by the focus on other symptoms) make this less likely.
Gastroesophageal reflux disease (GERD) or esophageal spasm: These conditions could explain the chest tightness and nausea/vomiting, especially if symptoms worsen with heat due to increased fluid loss or dehydration exacerbating reflux.
Anxiety or panic disorder: The symptoms of intermittent chest tightness, SOB, nausea, and vomiting, especially in the context of heat, could also be indicative of an anxiety or panic disorder, particularly if there's a history of such conditions.

Pulmonary embolism (PE): Although the d-dimer is only mildly elevated (2.37) and other symptoms like significant leg swelling are not mentioned, PE is a diagnosis that must be considered due to its high mortality rate if missed. The normal BNP and troponin levels do not entirely rule out PE, especially in a young patient with few risk factors.
Cardiac conditions (e.g., pericarditis, myocarditis): Despite normal troponin levels, these conditions can present with chest pain and systemic symptoms. The elevated AST and ALT could also indicate myocardial involvement, although this is less common.
Sickle cell crisis: Given the patient's African American background, sickle cell disease should be considered, especially with symptoms like chest pain and potentially elevated heart rate. However, the hemoglobin level (12.7) is not suggestive of chronic anemia typically seen in sickle cell disease.

Hypertrophic cardiomyopathy: This condition could cause chest tightness and SOB, especially with exertion or heat exposure. However, it would be unusual for this condition to present for the first time in this manner without a history of similar symptoms.
Mitral valve prolapse: Similar to hypertrophic cardiomyopathy, this could potentially cause chest symptoms, but it's less likely given the acute presentation and the lack of specific echocardiogram findings mentioned.