Could i still be pregnant?

Is it possible that I could still be pregnant even tho a baby didn't show up on my ultrasound when I was supposed to be 8 weeks bc I have a erfly feeling in my lower stomach. I have also had surgrey like 3-4 weeks ago, they would have seen it right? Oh my gosh its killing me bc it sometimes feels like pressure "down there" nd its kinda painful in my stomach sometimes.