I get this too, and have been checked by my GP and optician (which I recommend you do in case there's an underlying problem). In my case, neither could find anything wrong and I haven't been able to identify a pattern I.e. High BG readings.
Optrex make eye drops specifically for "bloodshot eyes" which I find helps with the soreness (not cheap). I'm my case, the problem lasts from a couple of days to a week and can look as bad as conjunctivitis. Annoying.
I'm inclined to blame diabetes, because I don't recall having this recurring problem before the condition. However, it is easy to blame diabetes for everything (my GP didn't!).