I often hear that if you have too long & low a position you can get back pain, but I never hear whether too short a stem can cause the same. I occasionally get back pain while riding (doesn't happen with every ride, maybe 1/3 or 1/2 the time). I'm riding a size 54 cross bike with a -7 degree 100 mm stem that is slammed. I am 5'-11" (180 cm) and very flexible, as I can place my palms on the ground with straight legs without warming up. I don't have the budget right now for a professional bike fit, and am wondering if it's worth it to try to swap out stems for something with a greater degree of drop & longer, maybe 110 or 120 mm. What do you guys think?

For me, yes. I commuted 5 years fix gear on a too large touring frame with a long headtube and laid back angles. Put a 130 (traditional quill) stem on it. Hills were painful I didn't take that bike for long rides.

One day I sketched that bike and stem. Over laid on that my Mooney and old racing bike. Saw that the bars on both were much lower and a little further forward. Got t hunch that if I had a stem made that put the bars roughly on a 30 degree line forward and up from those two bikes, I ought to have roughly the same reach and position. To hit that line would take a 180 stem. I gambled and had it made. Put it on and took a hilly 75 mile ride north of Seattle. No back pain! In fact I felt great all ride until simply being hammered started to sink in.

My current 6 stems are 175, (2) 140s, 135, 130 and 120. The 4 shortest ones are on custom bikes designed to keep the stem length to production and available. My analogy for bike comfort is the "cat stretch". (You've no doubt seen cats do that stretch were they get really long. My back loves that!)

Ben

Doesn't bother me but I get my handlebars wide and tall. Try ditching drop bars if that's the case...not hard for me with cruisers. Wald and Torrington (and bears) Oh My.

I don't ride an extremely stretched out position in general, but my lower back definitely will tighten up if my reach is too short. Stems can be had for cheap so it's worth a shot to try, but be aware that there are lots of other things that can cause back pain on a bike.

I'm going to say save your money and get a professional fit. I just did and it cost me less than buying a new mid-range aluminum stem would have. I'm about an inch shorter than you, both my cross and road bikes are 56c, and I run 100mm on my road bike and 90mm on my cross bike. I also ran my stem slammed, -12 on the cross bike, -8 on the road. Road bike has a 3cm shorter head tube, so my positions were as close to each other as possible given the different frame dimentions. When I got my fitting on my road bike he was surprised I didn't have back pain because from his measurements I had my saddle too high my stem angle too low and my cleats too far forward.

You can buy Kalloy Uno stems for $25. I have several, from adjusting my reach on different bikes over the years. That's the way to go. Try 'em out. In general, more reach is more comfortable for anyone of normal body configuration and flexibility, and of course both of those can be modified. I'm 72 and run all my bike bikes with -17° slammed stems and enough reach that my elbows are ahead of my knees with horizontal forearms. That's the least back pain for most folks.

Long top tube/ short stem, short top tube/ long stem . its the combination not individual parts..

Ever had to lift a lot of stuff for your job, or at home? may not be the bike at all..

I can tell you from my own experience that with a lot of people is that a back angle relative to horizontal of less than 45 degrees, or over 70 degrees works well for limiting back pain if fit correctly with the right amount of flexibility/inflexibility, this gray zone is the middle seems to cause a lot of issues as holding this ackward middle ground angle there isn't enough support provided by the hands and not upright enough to take the weight off the core. You can kinda try it yourself by sitting with the bike on a trainer with your hands off and moving the back through the range of angles

When the pain happens, it is on the bike only. Moment I get off the bike the pain disappears so I'm fairly certain it's bike fit related.

Another thing to look at for back pain is hip rotation:
Riding Position Discovery