I've recently developed a sore lump behind my left ear after having a cold and sore throat. it's about 1cm big and firm to the touch. serious?

Likely lymph node: It is most likely an enlarged lymph node, a sign that your immune system was working well to fight off the cold and sore throat. It should subside after a week or two. If it persists or grows or troubles you in any way feel free to get it checked by a physician in person.

Enlarged lymph node: Usually viral or bacterial infection cause enlargement of lymph nodes. Usually gets smaller within week to ten days.