I'm no expert on these things, but I would say it's most likely getting deprived of sleep and perhaps straining your eyes on HL2Rp
I'd reccomend a good week or so with good nights of sleep, and perhaps lay off or completely avoid your computer so as not to strain them beyond repair
My eyes were killing me last night, and like an idiot i played on the computer all day
Took me about 3 nights of full sleep before the pain left