When muscles are really, really tight for a long time the body lays down a fibro-fatty deposit over it as protection, often right over the muscle/tendon attachment to the bone. They move around a bit but are anchored to the site. They can get inflamed and, while rarely causing intense acute pain, can give some dull discomfort. If that's it, if you solve the inflamation, the disc may stay around but the pain disappears.
The alternative is that this is indeed a muscle knot, and a bad one. It's a spasmed part or whole of the muscle. It can give unlimited pain because of its rich pain nerve supply. It's 'stuck on full' and needs its sensors reset - which is no small job.
Heat is good because it dilates (opens) blood vessels. The body is fooled into flushing the area with fresh blood to keep the muscle cool, as it has a narrow temperature tolerance. The fresh blood carries lots of oxygen and carries away wastes, which alone can be a good treatment and can cancel the spasm if you're lucky.
Massage is great - for the same reasons. It also helps to stretch local and surrounding muscle fibres. Self massage is a great idea, or find a good Massage Therapist who you work well with.
RMTs, Physiotherapists, Osteopaths and Chiropractors are all good folks to see. They each have great tool boxes and can be used in combination to great effect.