How to heal lower back pain?

I'd suggest doing some self-myofascial release on areas that are commonly tight. Foam roll the IT band (this can also lead to knee pain if you've ever had any), TFL and hip flexors. Just "youtube" those terms to find out how to execute them. Stretch the hip flexors with a deep lunge position. Also stretch the piriformis because this may be irritating a nerve if the pain isn't muscular. If it is muscular, your best bet is to just rest it and possibly take some contrast baths (eg, hot bath, then jump into a cold shower, and repeat) which may speed recovery. Stretch the surrounding muscles as mentioned above because it may still aid. Also, try rolling the bottom of your foot pretty aggressively (never too painfully!) on a ball. I find this to sometimes loosen up the lower back. Do NOT actively stretch the back. The lumbar region does not need mobility, it needs stability. The tightness is likely due to other muscle imbalances or just overuse of the muscle.

Sorry to hear that I'm a frequent sufferer of the same problem. I find the only thing that works for me are either muscle relaxers or a hot shower or bath. I try to avoid doing anything to hurt myself even further, if it was myself I probably wouldn't compete as you know how important your back is. I say sit that one out at least until you're comfortable. Have it checked out by your doctor if it persists. God forbid you have some kind of herniation that is made worse by anything crazy.

Try doing some yoga to stretch it out and strengthen your back. I know it sounds weird, but honestly, it really works. Childs Pose, downward dog, and sun salutations are really good for targetting the lower back without being too strenuous. If you can find some good yoga on youtube, just follow that. Or if you look up the positions that will help too. Just make sure you alternate between them all to stretch everything. Other than that, keep a heat pack on it when it hurts. Also take some anti-inflammatories. That should help. Good Luck!