I've been riding since July 2012, crossed the 4k mile mark today, woo hoo! I finally got a fitting last month and he raised my saddle a full 2 inches and kept the handlebars at the same height. I never really broke 13mph before the fitting, then afterwards went out and did a 40 miler at 14.5 mph, it was amazing how fast the bike was rolling with the same or less effort. The honeymoon was short-lived however. I started having lower back pain and now, 250 miles later, I'm fighting it constantly. I feel like I'm falling forward and putting too much weight on my hands, although the saddle is now farther behind the bottom bracket than it was before. I'm wondering if the more efficient saddle position is allowing me to transfer more power through my lower back than previously, and my 42 year old back is having trouble matching it. If I slide back far enough on the saddle to get the weight off my hands, then I'm well behind the pedals and have to almost lock my elbows to reach the handlebars, unless I lean way over.
I've started yoga for core strength, but I'm only going to get older I can't get the handlebars up above the saddle like they were before because the stem won't go that high. I used to be able to ride 40-50 miles before I had back issues, but now, it's constant, although I've picked up a full mph on my average speed.
I'm hesitant to lower the saddle, but feel I may have to. Any advice?
Thanks,
Jon

Yoga, for core strength? That will take years. Crunches will start to return results in days. Are these flat-bars or drop bars? How far from the saddle nose? Don't they have hills in ATL? Ride down one, you'll break 14.5mph easily. If you can do a half century @ 14.5mph you don't really need my advice. But I wouldn't lower the seat just yet.

H

2" is a lot, so obviously your saddle was much too low previously, so going back to that is not a good idea, and you saw positive results, too, so I'd scratch that. Back pain-- any pain-- is a real problem, though, so you need to do something.

I think looking at bar height is a good place to start; if the stem cannot go higher, have you considered replacing it with one that has more rise? I'd probably suggest starting there, trying to replicate the bar/saddle relationship you had before, so that you can either identify the angle of the back as the culprit or not.

It really could be any number of things, and working off just a text description leaves a lot to speculation. I mean, I presume the fitter is competent, so it's likely not any of the gross adjustment stuff. You really should go back to the fitter and get their input on this.

If, for whatever reason, you want to chase this down here, some profile pics of you on the bike at the bottom of the stroke, or even better, a video of you pedaling the bike on a stationary trainer, also taken from the side, would be the most efficient way to tackle the issue.

This is all good advice, couldn't add anything intelligent if I wanted to. But I am surprised you were riding with a saddle that was a full 2 inches too low. Was your saddle and bars the same height before, and now the saddle is 2 inches higher than the bars? For sure that could cause back and neck pain.

The bar flats were maybe half an inch above the saddle before, but the stem is not long enough to get them too the level of the saddle now. I emailed the fitter for a follow up, waiting to hear back. I'm thinking that he may have fit me into a racing type position that my body cannot sustain for long periods. And at the new saddle height, the bike frame is too small to get the bars up where I need them. So I may be looking for another bike I think

With that much of an increase, I usually mark the seat post with the final position, raise it a half inch and tell the customer to raise it another half inch each week. that's too much to do at once. Work up to it gradually and your muscles will go along with that.