Hello,
yes, it is possible to fail the second test after passing the first test. So, not a bad idea to be checked.
Being diabetic is not necessarily a reason not to have a natural delivery. Please work hard at diet and avoid sweets in the next few weeks so that baby won't be over fat. 3 weeks can still make a difference.
Also, you should know that ultrasounds are not very accurate at predicting weight at this stage of the pregnancy.
The doc said my baby was roughly 3.5kg (7lb 9oz) already that's up from 5lb 3 weeks ago and still 3 weeks to go