Thanks for the query,
Based on your description, I can think of 2 possibilities:
1. The hard lump under the scar could be a HYPERTROPHIC SCAR/ KELOID. This can appear following an injury
to the XXXXXXX dermis
(skin) and any injury heals with inflammation and scarring. Most hypertrophic scars
may undergo partial spontaneous resolution, but it might take a couple of months.
Is the scar /lesion red in colour, any associated itching, tenderness/pain, is the lump elevated above the scar?
2. Localised induration of the underlying structure can occur as a result of inflammation. This can also cause similar hard lump.
Though both are not serious concern, I encourage you to consult a nearest doctor for a physical examination. I am sure your doctor will be able to differentiate the lump and relate it.
Hope this answers your query. Let me know if you have other concerns.