There's a lot of it about!!! This is about the 4th similar thread in 3 weeks. I had very similar problems and it came on very suddenly (and went away just as quickly, until the next time). As well as all the stretching, the thing that seems to have sorted it (fingers crossed) is massage. I had an aromatherapy massage last week and then a self massage (as in this months RW) and did a 10 mile PB (after 9 years) on Sunday, when 3 weeks ago, I couldn't do a mile.
Good luck