Ultrasound

So I just got home from the obgyn. I'm ten days late for my period. All negative pregnancy tests. On the ultrasound, she said that the lining was very thick and there is something in the lining. Either a baby or polyp. She said since the pregnancy tests are negative, it's most likely a polyp. What gets me is that I've had about 3-4 ultrasound this year and not one was there a polyp. She gave me provera to jump start my period but I'm kinda skeptical. What if it's a baby. What do you ladies think?