lower back pain, right side during cycling

any clue what causes this? too much water? not enough water? it cant be too much water because I was making sure i wasnt over doing it, any clue?
it gets even worse when I breathe in really deeply.

Are you right-handed, and does your bike fit properly? Also, how's your core strength and overall cardiovascular fitness?

It sounds like a muscle balance issue, especially given that it hurts more when you take a deep breath in.

You should have your fit and pedaling form checked. Core stability (as mentioned above) is an important factor. You may even want to see a PT, a McKenzie trained therapist would be my first choice.

It's very unlikely to be a metabolic/water issue based on the unilateral nature of the problem and the pain with breathing description.

I have lower back pain sometimes after long rides. I'm fairly sure it's core strength weakness I need to work on. Just a thought.

For me, lower back pain on the right side is related to a sacroiliac problem I've had for decades. But it's very low, hip-level, not affected by breathing. FWIW, regular stretching, especially of the hamstrings, helps it a lot.

With my old road bike, I used to get mid-back pain related having to hunch my neck and shoulders in order to reach the handlebars.

I get that sort of pain toward the end of long rides. It's mostly because my core is weak, and I'm very right-sided. I end up putting much of my weight on my right side (which is why my right wrist hurts right now and the bar tape on the right side is all messed up!), so those lower back muscles end up overworked.

I used to have that kind of pain on my right side, but after my nine broken ribs were entirely healed, it seems the pain has gone away.:D

I've had that happen and when I got my fit checked it turned out one of my cleats had slipped out of place.