Hi, Thanks for writing in. The cerebellar tonsils are the most inferior portion of the cerebellum, a part of the hind brain which is important for motor coordination. Normally, these tonsils are wholly inside the skull. A 3 mm low lying tonsil does not clinicall fit into the abnormal category. In the case of Arnold-Chiari, the tonsils are low-lying often more than 4 mm, below their normal level. It can cause severe headaches. The presence of a syrinx in thee cervical cord needs to be ruled out and that will be mentioned on MRI should it be there. Cervical nerve root compression can also cause headache on lifting weights. Please discuss with your doctor.