Heidi - I first experienced this about 2 years ago wen kneeling on the floor grappling with my then 3 year old (also called Heidi!) in Ikea! Now I get it every time I kneel on the floor. It's never turned into anything serious but it is a very sharp pain when it happens. I have trained for marathons since and it has never given me cause for concern when running or indeed doing anything that isn't actaully kneeling on a hard floor!
Rich - my pace fluctuates a bit too but I put that down to the routes I train on rather than heat/humidity as I do all my running at the same time of day - very early! I ran 40 miles last week, will be doing the same this week and then it should steadily increase to the high 40's over the next 12 weeks.