it may be a couple of things, but you should have a compartment pressure test of your forearms as soon as possible to eliminate compartment syndrome.

If you have eliminated compartment syndrome then you will need to evaluate your riding style, upper body strength, lower body strength, core strength (abs/low back) and nutrition/hydration.

You appear to be working hard off the bike but you may be doing exercises that are causing you problems with your forearms.
Let me know.
Alan