I was at the beach last july and stepped off a walkway, the step was very deep and when my left leg stepped off, it hit very had, I had immediate pain and it was very hard to walk. It slowly got better, but I have had on and off pain in the back of my knee since then. It is usually just an ache. Today I have been out all day running errands, walking on the hard surfaces of stores, (walmart). Both legs have been bothering me tonight, but my left leg, that was injured, is very achy. It is not a constant pain, but intermittent. I have taken Aleve, which has helped a bit. I was concerned if this could be a blood clot or if it is just from my day of lots of walking and the previous injury.