ectopic pregnancy?

Aly
So I had severe cramping today that kinda felt like horrendous gas that I couldn't get out. So I went to the doctor and he wanted to do an ultrasound. In the ultrasound they couldn't see the baby and he said it could be an ectopic pregnancy or just simply be too early to see the baby.. anyone have this or a similar experience? Also there's no spotting or bleeding. I'm about 4 weeks today.