When i cross my arms, or exert extra pressure on my legs by pushing up, my chest gets tight & feels like something is sitting on it. is this muscular?

Valsalva maneuver: This increased chest tightness sounds like you are describing a valsalva maneuver. When you exert extra pressure through your legs by contracting your muscles, your core trunk muscles also tighten. There is limited space inside of your chest due to your rib cage, so when pressure increases in your abdomen, the abdominal organs also push up on your diaphragm and cause this feeling.