I don't believe for a second that it can't be detected early on because of Diabetes. I've been pregnant twice and the tests from the shop came back positive both those times when I was only 3-4weeks pregnant.
You'll never really find one answer for this. You'll likely not pregnant, it's probably your body reacting to the stress and suddenly getting enough insulin again, etc but we can't tell you that for sure because a) we aren't doctors and b) we aren't you.
If you REALLY think you are, then go to another doctor and have them do the test..though it's not much different than the tests you can get from the shop. I've been told they use the same tests you can get from the store.