Is there anything that can build up in the body and make you feel ill- weak and body aches- that blood tests can't see? doc's can't figure me out

Depends: Most diagnoses are made from the history and exam findings. The blood tests are used mostly to confirm the diagnosis or assess its progress. On the other hand, one needs to tailor the type of blood tests to the disease condition suspected and thus merely saying that blood tests were unrevealing is not too meaningful. Some symptoms are psychophysiologic and cannot be detected by a blood test.