So I went to the hospital last night for pain and swelling in my foot. well the doctor came in and…

So I went to the hospital last night for pain and swelling in my foot. well the doctor came in and told. me well I dont know whats wrong with your foot but your pregnant. So they did an ultra sound and could not find anything. but two urine test and an Hcg of 60 confirm pregnancy. Based on last period I am 5 weeks. is it normally to not see anything on an ultra sound at this stage?