Does my brain have limited learning capacity or memory?
Short answer is yes, the brain has a limited capacity. It's a large but limited physical system with numerous but countable connection, it follows that it must have a limited capacity. 

Learning wise, we usually say we learned something when we can recall it at will. I'm not sure I agree with that definition, but if this were the case, learning would be intimately related with memory capacity, so the answer would also have to be yes. 

All this without taking into account energy consumption and metabolism which would impose also further limitations, e.g. I bet you that if you switch the activities around and start reading in the morning and try to work at night your work would suffer but your retention capability would improve (just a guess). Now, is all this relevant? I can't say, we (humans) might be working already at full capacity, in this case it would be relevant, or we could be very far away from this theoretical limit and all this is moot.

If that was really your question then you're done. 

However, you seem to be preoccupied with a mismatch between the expectations of your own retrieval abilities after a particular "type" of activity and what you can actually remember. 

I would be remiss in I didn't say that I agree with WYSIWYG's comment, I'm sure this is not the place for health advise. So with that in mind my 2c: 

your expectations can be misaligned, that's always a possibility; your retrieval abilities can be impaired, which means that you learned fine but cannot recover it; or may favorite: you might just be tired! IANTTOAD