Was treated for strep throat 3 weeks ago. Tongue still painful now and I noticed a growth way in back on tongue. Loss of taste. Burning on tongue.

See md. Go to see your doctor and get yourself checked. Especially ask to examine the growth on the back of the tongue. Your doctor then can refer you to see an ear nose and throat doctor for further examination including a biopsy if the growth is confirmed and looks suspicious.
Possibly yeast. If you did a course of antibiotics you may have now developed a yeast infection in your mouth. It would not likely have caused a true growth but you may have inflammation of some of the taste buds which can appear as a growth. You need to see your medical provider to confirm the diagnosis and get a prescription for an antifungal mouth rinse.