I would call my doctor and explain what's going on. They can have you come in for blood work before your appointment. The bloodwork will test your progestrone level and hCG levels. They should have the results by the time you come in for your appointment, earlier maybe. If your progestrone is low they can put you on supplements until the placenta is formed.
Depending on how aggressive your doctor is, you may have to be pushy. If you want the tests, and they tell you they aren't necessary, explain that you're very worried. Insist on them, if you want them, but it shouldn't come to that. These aren't unusual tests, but they aren't routinely done for every pregnancy.
It probably is just dehydration. That can be a common cause of spotting and cramping. I'm overly cautious about these things, because I've had miscarriages. It's no fun at all, but they're more common than you'd think.