What causes tight/crampy feeling in back of legs, mostly after excess standing? no noticeable swelling. slight ache in knees also.

Calf pain: Likely muscle soreness or strain. Try stretching leg intermittently. Do you keep your self enough hydrated?

I would also : Look to see if you are flatfooted or hyperpronated as excessive standing on this foot type strains the posterior muscle groups and the knees.

Imbalance: When standing for a prolonged period without a break, it does put a lot of pressure on your leg muscles; if you wear heels, it puts even more pressure on the backs of your legs because of the added angulation involved; you may want to wear flatter shoes, or find ways to move around occasionally, so as to change the pressure on your muscles;