Pain in left chest when laughing hard, & sometimes breathing in. Is this bad/serious? Idid some diff exercise 2day & yesterday. Could it b related? Ok?

Strain muscles. Nothing serious at all.When you exercise(certain types)you are using the same type of muscles that you use when laughing, if they are strain from exercising they will hurt when laughing.Pain should go awway in a couple of days.
Costochondritis. Sounds like costochondritis. See if you can reproduce pain by pressing on the area. This pain is due to inflammation of the cartilage between the ribs and sternum. Anti-inlammatory meds and rest are the best treatment.
Sounds like muscle. Chest wall pain from exercise. If you press on the area and it mimics the pain, then it is muscular and not worrisome.