Constant chest pain under left breast wheni breathe or lay down. very tender to lay on or touch . worsens when coughing or laughing?

Several things: This is a problem that necessitates a face-to-face meeting with your doctor. This will allow him/her to examine you, ask specific questions. And possibly order tests to find out what's wrong and what to do to help you.

Costocondritis : I suspect this is either costochondritis - inflammation in the joint between your sternum and ribs or an intercostal muscle tear. Less likely is a rib fracture. Much of this depends upon your history leading up to this. Have you been coughing? Have you been doing heavy exercise or lifting? Do you have asthma?