Melanoma in situ, tigh, WLE a week ago. Now pain in the tigh and groin, can feel a lymph node. Is A reaction to the surgery? Or melanoma spread?

Needs monitoring. You are right, it may be a reaction to recent surgery or(less likely) there can be melanoma spread into your lymph node. Only time will tell you which possibility it is. If the nodes are enlarged due to reaction to surgery(or infection), then they will settle down and decrease in size in 4-6 weeks. So keep an eye and check the size of the node once weekly and see if they decrease or increase .