Forearm pain during and after cycling
There are multiple possible causes for this kind of pain.

Of course, you should see a medical person in case something more serious is going on than our amateur diagnosis suggests.

In general, this is a kind of overuse injury. This can have different causes (and each would have slightly different symptoms). Part of the reason is that you've taken a break and those muscles are just not as strong as they used to be. Common causes are


The most common cause is your seat too high relative to the handlebars, leading you to put more weight on your forearms. This would be more common for mountain bikes and flat-bars, and seems more likely given the posted diagram.
Another cause to gripping too tightly. You could be gripping the handlebars too tightly, or trying to do too many jumps, etc. This can also be due to repeated hard braking.
You could be overstretching the tendons / muscles, by riding with your wrists low so that your hands are higher than your wrists, or with your wrists bent to the side so that each wrist is bent inwards or outwards. This is more likely on a road bike if you ride with your hands "on the corners" a lot. Or perhaps if the handlebars are too narrow.
Even stranger, it can be caused by pushing too hard on indexed gear levers. You would probably know if your enthusiastic gear changes were causing it. 


EDIT

Often the solution to this kind of problem is suggested by the diagnosis but requires some experimentation because the the result of a change can be hard to predict. Everything is interconnected, so changing one thing without full understanding can have unintended consequences. If you make changes yourself then be vigilant for other issues.

Taking the bike to your LBS to get them to check your position is recommended.