Hi it was 6 months ago i had a total knee replacement is it normal now for lower leg to throb constantly am exercising it but no relief?

No: No that is not a "normal" complaint and there are many possibilities. I would not hesitate to be seen and fully evaluated for it.

No not normal: Need to work out if soft tissue vein problem or something more troublesome like loose components. Recommend GP assessment as may need tests done to sort through possibilities and work out treatment

No : Throbbing in leg cannot be related to your knee replacement , there has to be another reason for that .

Leg pain: In a ideal world it should not be sore constantly . More info is needed. With respect to duration is symptom, new injury, mode of onset, swelling at joint or calf etc . Please see your doctor for examination of joint and review