Hi I'm a 46 year old woman who was diagnosed with soft tissue sarcoma in left foot amputated below knee. What does the swollen lympnode left side neck?

Not related. Soft tissue sarcoma do not metastasis via lymphatics except in very rare situations. Thus these lymphatics are not related to the sarcoma you have. I advise you have them checked by your pcp. Good luck.
Likely nothing bad. Likely, this is just an infection. Most sarcomas spread through the blood stream and not lymph nodes. When they do spread through lymph nodes, from the foot they would go to the groin. That being said, if the neck lymph node doesn't go away in a couple of weeks or if it grows quickly, see your oncologist just to double check. Cancer doesn't always play by the rules.