On/off pressure n pain in sternum area for about 2 days.worse when lying down n pressing it.is it due to straining muscles from a few days before?

Very possibly: The things which exacerbate or trigger your pain, such as pressing on it, favor a musculoskeletal (joint/tendon/muscle/bone) cause. Also, given your age more serious heart/lung causes of pain would be less likely. Often one can't recall a specific activity causing the strain, but when you can, that helps support the diagnosis. Rest, use ice if you want, take anti-inflammatories. Better in few days