I have a rash on my neck for about 1 week red blotches woke now a rash on my upper right side I have been very tired and taking phentermine?

Drug rash. Based on what we know, the most logical step would be to stop the Phentermine and see if the rash goes away and stays away. Any drug can cause a rash. Otherwise, someone will actually need to see the rash in order to be able to help you.