What causes dull ache pain right below the left chest area
made better with heat
Have you done any lifting recently? I say muscle pain. Sounds like you pulled a muscle there. A sharp pain might imply a cracked rib.
It might be a spleen, no wait that's the right
If you are over a certain age and female then it might be that time of the month.
It could be a pulled muscle, cramps or something else. If it continues to get worse see a doctor but I don't think it's serious