Hi i'm a 4 weeks pregnant i'm having no bleeding or camping my hcg levels are low. they did do a ultra sound didn't see anything in my tubes or a sac?

Too early: It may be too early for them to have saw something inside the uterus. If your pregnancy test is positive by blood or urine than you are pregnant. It could just be that you need a little bit more time before able to see something via ultrasound. One option would be to repeat the carmona pregnancy and 48 hours by a blood test. Another option would be to repeat the ultrasound in one week.