From what you've written it seems to me as your horse is working in a long low outline which is good = acceptance of the bit & relaxed but i could be wrong.. but it wouldn't be your hands being too heavy as the horse would most probably become more hollow with heavier hands :) Hope this can help.
In answer to your query, It could possibly be a girth gall which is common with elsaticated girths as they can be done up too tigght without anyone being aware, It may be other things but this sounds like a girth gall, I'd use a girth sleeve for a few days and avoid doing any work which will...