How early can a doctor tell your pregnant?

I'm 18....i'm about 3 days late on my period and i'm NEVER late for my period...I had unprotected about 3 weeks ago so it is def. a possibility that I am pregnant but my ques. is.....I went to the dr. today to get some tests done for kidney stones and a UTI but they did an ultrasound on me and an internal exam...but since i'm only 3 days late and they weren't specificaly looking for pregnancy would they even be able to tell yet if I am??????? I haven't gotten the results back yet...and I didn't say anything because if I did my mom would find out and I don't want anyone to know yet....so would they be able to tell?