Sciatic like pain stiff sore knee.no visible redness/swelling.when to worry about fluid on knee or something bad?just saw dr for calf pain.blood fine.

Don't worry: I would not worry so much as just that you might want to be seen for a specific diagnosis and treatment options. As you mentioned, it can be a variation of a sciatica, which is leg pain caused by an irritation if a nerve in the back; or it can be from something in the leg itself. A detailed exam and appropriate studies will likely yield the correct diagnosis.