Why Is My Stomach Firm And Slightly Rounded Post Menses?

My stomach is feeling firm and looking slightly rounded, I've gotten my "period" but they were lighter than usual so it's possible that it could have just been vaginal bleeding. I took an 8$ pregnancy test and it came back negative but my stomach still feels firm and bloated. Is there any way that i could be pregnant? Im 19 years old and had unprotected sex once on november 12th and again on January 21st.