this may not be any cause of worry, if u are getting positive results then ur pregnant and the reason it wouldnt be picked up on a scan is because ur probably too early, i had an internal scan yesterday as i was also bleeding and the lining is thick as it should be in pregnancy and it was again confirmed at the hospital that i am pregnant, but in the scan they couldnt see a baby, because im very early, like 4 weeks, the bleeding is very common, its implantation bleeding, when i first did a test it was faint, i did 3 the same day all faint, and then when it was confirmed at my doctors early last week it was weakly positive, i did another test to check my hormone levels last saturday and the line was much stronger, but its still too early to detect on a scan.
so have u had confirmation from ur doctor? if not go get that done, give it another week then they should deffo be able to tell