I have a feeling of tightness, fullness like a knot just below belly button?

Some possibilities..: Feeling a knot of tightness near the belly button... Probably not in the skin, so the next layer is the muscle... Maybe a muscle strain from sit-ups or other exercises. Not likely to be a hernia there unless there is a bulge. Under the muscle layer are the intestines... Maybe some intestine contraction or spasm, due to no particular reason or to constipation. A doctor's visit can sort things out.

Could be many: Things from constipation, to bladder retention, diverticular disease, if you are female can be uterine, only a more detailed history and physical exam can help pin this down.