You would have been 5+1 Monday when you had your scan? Are you definite about when you ovulated? It's possible if the 28th was a proper period you'd only be just shy of three weeks pregnant, you may have ovulated quite early on and this is why you're seeing a positive now. Or it could be that that wasn't a proper period and was an early pregnancy bleed, in which case there's a chance if you were only just about five weeks they may not have seen anything, more so if you're not definite on when you ovulated before, you could be a little earlier. To add, it's also possible if they only looked at your ovaries they may have missed anything else completely