Refrain from lifting. Don't forget to exhale as you lift, but would refrain from lifting until you get the chest pain checked out by your md.
Possibly serious. If your chest muscles are sore to touch or the pain can be reproduced with certain positions or movements, it's not serious and will improve with rest/time. But if you get a tight, heavy, or squeezing feeling deep in the chest with exercise such as walking or lifting, it's time to see your doctor as this may indicate a more serious problem.