Usually if your doctor won't give you your blood test results over the phone, it means that something has come up in the bloodwork that they want to see you about. What was the blood test for? If it was for something like an infection or an STD, they may want you back so they can prescribe you the proper treatment for it. It could be a variety of things, depending on what you were being tested for.
On the other hand, it could be something as simple as them having contaminated your blood sample. The last time I had blood drawn they called me back into the office, only to tell me that someone had accidentally stuck a finger in my blood sample and they had to take another.
Good luck, and I hope it's nothing serious!
