Lower back pain

[Hell and High Water]

Having an issue that I thought someone might be able to help on...


I have an older/used bike that's in very nice shape. The LBS did a bit
of a fitting when I purchased and the bike feels pretty comfortable.
The bike came with a metallic quill stem, which the LBS guy thought was
a bit short for me. I bought an H2O stem which was a bit longer, and
has a bit of an upward angle.

My problem is low back pain.

While I'm riding I don't notice any pain, but either later in the day,
or even the next day, I feel pain right where my butt meets my lower
back. Sacroiliac joints? (sp?)

I have found that if I put my leg up on something that's about waist
high, like the bathroom sink for example, and really stretch out the
backs of my upper thighs, it really alleviates the pain.


Almost feels like my hamstrings are super tight and pulling down on my
hips?

??

Not sure if that's even possible.


Anyway, I'm wondering if anyone has experience with this. Is this a
fitting problem?

Posture?

Riding style?

Thanks for any thoughts or suggestions.


-Bianchi Bob