Na, I know what lactic acid build-up fields like, and it wasn't that. It was literally as though my muscles were pulling so tight, so they felt like they were at breaking point. Stretching them helped, but its taken two days of non-running rest for them to work themselves out a bit. Saturday they were still tight, Sunday they were tight, but loosening, and on awakening today they had relaxed... so I went for a slower run and stretched them well, and they still seem ok at present. I was pushing pretty hard over a week ago. Last week when I had the issues with it, I slowed right down to easy jogging pace, but prior to that I had been stepping up my running to sprinting by the end of the 5km.
I think I've just been really hard on them, and considering I'm nearly 43... I've resigned myself to the fact that I just can't punish them as much as I used to be able to and get away with it.