Sports injury lower back pain. Sports.

Hi I m having lower back pain for about 6 months but I used to play with that back pain for some days then I visited a doctor he told me to take X ray the X ray was normal then he further suggested me to take MRI in fortunely my mri also showed normal then I was advised to take physiotherapy IFT then I started to play cricket after a month then I felt some gradual pain which affects me a lot after playing but it heals after a week then I continue to play and the physiotherapist also suggested to do some core strengthening exercises but I did only for a month then currently im experiencing pain my question is can I now join a gym to strengthen my core and is it advisable to take steirod injection after knowing the root cause of the back . My pain is exactly on my lower back to starting of my glute . So can I join some gym to strengthen or shall I take a steriod injection by examining the root cause please help me solve my health problem . My age is only 20 so I can't leave my sports .