How early can ectopic pregnancy be detected?

Hello, this is my first pregnancy and I am only 4 weeks pregnant (17dpo day 4 of my missed period). I am feeling some dull cramps on only my lower left side and I’m concerned this could be ectopic. I want to go in and get an ultrasound to check but i’m not sure if it’s too early for the doctors to even see anything and don’t want to be billed and have them tell me to come back in a couple of days. Has anyone had an ectopic pregnancy that’s been detected at around 4weeks?