I had ultrasound done on my thyroid. Nurse said that I have a 3.8cm mass, she also said cyst but isn't sure hypo eccentric. Could it be cancer?

Wait. And get the final findings from the radiologist, not the nurse or technician doing the ultrasound. Confirm the size is in cm and not mm. Thyroid cysts are generally benign. See your doctor who ordered the test to follow up with the final radiologist's report ; their recommendations for possible treatments.