What causes my swollen , hard, and painful gland on the left side of the groin?

Swollen painfull gla: It is better to see your PCP-for evaluation and assesment and to r/o any infection.

A Hernia: If the swelling disappears when you lie down and massage it, it may be a hernia. May require simple surgery if painful. If not need to be evaluated by your doctor.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.