I've been in a similar place with back pain in my lower back. I also play a lot of sport. Simply a mixture of not stretching enough after sports combined with s

I've been in a similar place with back pain in my lower back. I also play a lot of sport. Simply a mixture of not stretching enough after sports combined with sitting for long periods of time and not being active enough during development hours can make your body really tighten up. My advice is to see a good physio even just for one session who will most likely be able to manipulate your back etc. and really help you out. That's what I did and it was the best thing to do. I'd left it til it got that bad I went to hospital because I thought maybe there was something really wrong... a few days later and one session with a physio I felt awesome. Riding a bike is great cardio for you and it shouldn't harm you. It's just making sure to stretch out all the right places after. It's amazing how many places will affect your back.