My pregnancy tests showed a weak positive result 4x and a negative onei had had an ultrasound but there is no sign of a fetus.Why is this?

The only reason you would have a positive test with no fetus means either it's a somatic pregnancy (in your head) it's ectopic (in you fallopian tube) or you have cancerous tumors in your liver. Get a HCG Beta Subunit (ask for this) from your doctor. It's a blood test to determine your exact HCG level