Ok so today I am 10 days post op. About three days ago I started having this weird pain like sensation in the back of my thighs, back of knee and sometimes back of calf area. It only happens when I stand up from sitting. Sometimes it is so intense that I feel like my legs are going to give out. It only lasts a few seconds after I stand then goes away. Any ideas what this could be?

Call doc soon. I was told if I had ANY weirdness or pain in my legs to let them know ASAP. They worry about blood clots in the legs which can occur after surgery especially in the overweight.

Yep - call your doc or go to ER. Blood clots are not to be messed with.