Pablo.

I recently flipped my stem down. Any other factors that could be contributing to this?

carpediemracing 

The problem is you have no descriptions. I'd consider a lower back issue an issue if I can't straighten back up to a sitting or standing position. I don't know if you have the same standard for defining "issue". Another BF rider complained of back soreness after enormous efforts or an ungodly amount of climbing - that would be perfectly normal, and in fact, if I'm racing, my back gets a bit fatigued within 60-90 minutes, quicker if the race is harder.

Pictures would help since it might be that you lowered your bar position from 5 inches too low to 7 inches too low.

However, as a general shotgun response, trying to hit everything in the area...

Any changes to your normal day to day routine can cause soreness etc. For example, if you started lifting weights for the first time ever today, you'll probably be sore tomorrow or the day after.

You changed something on your bike, it'll cause some stress. The stress, in turn, forces adaptation (stronger or more flexible muscles) or causes failure (if it's extreme - so if you lowered your stem really far and blew out a disc in your vertebrae, it caused failure).

Finding your optimal position, as you are discovering, is like trying to hit a moving target:
https://sprinterdellacasa.blogspot.co...ive-thing.html

With more info I think you'll get more info.

hope this helps,
cdr

big john

A lot of us have back pain or soreness while climbing if we haven't been doing a lot of climbing, even if the mileage has been high.
Like CDR said, the bar position is hard to decide sometimes. Over the years I have raised my bars as my back has complained.

jfmckenna

Fist of all why did you flip the stem?

Climbing is hard on the back.

aham23

strenghten your core and give the body more time to get use to the new position. later.

Hammertoe

^^^ Yes...

And work on your flexibility....


You get more leverage when climbing with a higher handlebar position...

LifeIsSuffering

You'd get more responses if provide further details.

Some examples might include:

what is your stem angle now and what was it before the flip?

do you stretch on a regular basis or just when the eye candy is good at the local yoga class?

how often do you do core work off the bike and, if so, what does it consist of?

do you climb mostly in the saddle (sounds like you do)?

have you had any lower back issues in the past and, if so, what type?


******************************************

The only guess I can give right now, based on your first post, is:

the sharper stem angle has placed your lower back into a more compressed position (and staying constantly in the saddle makes it worse)

your glutes and hams aren't used to the new body position; you may need to give them time to adapt - like CarpeDR stated

you're probably making the mistake of never getting out of the saddle

Like I said, however, more details will elicit more responses.