8 months Postpartum, not the same energy as before

Hello, I am 8 months postpartum and have started working out 5 days a week. I am at 150lbs and target weight is 135lbs.

But my issue is I have no energy like I had before the baby. Right now I am on mat leave for a year, but before my son I had a full time job where I walked about 3-4 kms everyday and I was able to hit the gym too. Lately I have noticed I feel tired very soon And need to take more frequent breaks from the gym, back hurts like a b****h. I just want my stamina back. Is there anybody with similar experience? And how did you overcome it. Thanks.