I developed a bump in my lower back on the left side above the sacrum, it's only sore when pressure is applied.this happened after deadlifts at the gym

Suspect muscle: This could be a cyst which may or may not be infected. With this there could be muscle or bone or joint or combination with tendon and bursa involvement as part of the pain. Sometimes at a gym there's risk of infection. Sometimes if you look at the bump there may be a small hole that may be plugged indicating a possible inclusion cyst.