Darcy makes a very good point. I've had a similar situation recently. For the past few years, I've frequently gotten stiff/sore shoulders both sitting at my desk and when riding my bike. In January, a stressful day at work triggered a very painful muscle spasm in my right trapezius (where the shoulder meets the neck) that affected my arm. I ended up getting x-rays and going to a chiropractor, who found that I had some serious mis-alignment in my cervical (neck) and thoracic (mid-back) vertabrae. As a result, I'd picked up postural habits that actually excacerbated the issue. Over the course of my treatment, I've learned a good bit about what Darcy's referring to regarding discs, inflammation, and how the nerves can be affected.

Two months of chiropractic treatments have made such a difference. At this point, it's been weeks since my shoulders last stiffened up while sitting at my desk. I haven't been cycling since December (which is just as well, considering the muscle spasm issues), but I had a fitting this week and noticed a significant difference in how my neck and shoulders felt while I was pedaling the fitting bike.

As Darcy suggested, be careful in deciding whether what you experienced was just a muscle ache or something possibly spinal-related. It couldn't hurt to talk to your doctor to find out whether it's something that needs to be treated. And, either way, the body does change over time and as we use it in different ways. It's never a bad idea to have your bike re-fit periodically to adjust for those changes.