several hard, immovable lumps on the bottom back of my head.

Welcome to e health forum.

Without a visual examination, it cannot be confirmed about the exact cause of the lumps.

Multiple small lumps in the scalp can be due to conditions like
1. Sebaceous cysts,
2. Infections of hair follicles (impetigo, Carbuncle, multiple abscesses),
3. Infiltrative skin deposits of fat tissues (lipoma),
4. tumors like neurofibroma.
5. Seborrheic dermatitis, psoriasis, eczema can also present as multiple lumps on the skin.
6. Fungal infections like Tinea Capitis

Hence for a proper diagnosis and proper treatment, you might consult with a dermatologist or your doctor for a proper advice and help.

I hope this helps.