Clyde1820

Some areas to consider ...

Not enough muscle strength or flexibility.
Over-use.
A strain, for any number of reasons. (Such as too tight of a grip, "stiff" upper body instead of relaxed, etc.)
Inflammation in the area.
Something was strained and you're not affording the body sufficient recovery to "fix" it.
Fitment, with the bike. (Something bent, shorter one one side, etc.)
Technique, with you. (Favoring a side, increasing strains on some muscles over others, etc.)
Nutritional deficit, though I'd think other zones would also ache similarly if that were true.

Hard to know, for certain.

Used to have occasional aches and issues with serious distance running, back in the day. Did lots of hard cycling and swimming in those years, as well. Had to vary what I did, regularly, to ensure I challenged all of the muscles in different ways and didn't over-use, over-strain or get "in a rut" with certain muscles or joints through too great a focus on any one thing. During most weeks, I'd find a little adjustment, tweak, stretch, or other change to my training was required to ensure I could perform in peak condition, strength, speed. Eventually, throughout the season, there'd be benefit from taking off-time from the serious training and instead doing much-reduced intensity activities for several days (or longer).

I'd get back to basics and check a few of the simpler things. Take several days off the cycling, as part of recovery. Do other physical activities for a couple of weeks. Do a bit of stretching in the area; work on flexibility. Get the area massaged, daily if possible. Verify your fitment, riding position. Might be that something's bent of out of whack with the bike's setup, causing one side to be showing strains before other areas. Once you get back to frequent riding, go slowly to ensure you're not inducing strains and stresses.

If it persists, it might be worth running by a sports-therapist type instead of the "family" doctor.