Hi , I Am 19 Years Old Healthy Eating Girl.

Hi , i am 19 years old healthy eating girl. Ive noticed that when ever I tax myself physically (by this i mean running a couple of flights of stairs) it seems like my heart flutters or maybe skips a beat (no idea how to explain it). If i do a little bit more I may experience chest pains (in the area where my sternum is). Also if say I sit up or bend over quickly my vision blacks out and sudden rush of blood through the head seems to happen and I feel disoriented. What is happening?! Thank you!