What Causes Pain In Sternum Area?

Hi, my name is amanda. I am 19 years old. Last night i was hanging out with my boyfriend, and he is a lot more muscular and larger than i am. He had been laying on top of me alot, and after getting up i had terrible pain in what would be like right under the sternum extending into my back. My stomach also hurt and i got pretty nauseous when getting up. There was no moment of him putting a ton of weight on me that made me feel pain, it just so happened that after getting up i did have the pain. It is now hours later, and i am still having an aching pain in my sternum area and a little below, plus nausea. Do you have any idea what this could be?