Hi Nicole, I know your mind is probably racing right now and I hope this gives you some peace of mind: it's very, very, VERY common for ultrasound machines to be unable to detect a fetal hearbeat prior to 8 weeks. For this reason, my OB office won't even see you until you are 8 1/2 weeks along. They've had too many patients get freaked out for no reason. I actually have a friend who saw no heartbeat at 7 weeks at her OB's office, went back for another ultrasound at 9 weeks - and there it was! She's currently 22 weeks along and everything is just fine.
I agree with Batts, if you'd miscarried, you would most likely know. And normal period-like cramps are also very common in early pregnancy. I had them with both my pregnancies for the entire first trimester. I'd run to the bathroom, thinking for sure I'd see blood on the toilet paper, and nothing. It's just your uterus growing and stretching to accomodate your new baby.
I know it's hard to not let your mind wander to the worst, especially when you're dealing with other medical issues like diabetes. I hope you're able to find a way to relax until your next ultrasound! And btw, congrats on the pregnancy!!! :)