Every time i have a bath i get itchy after i've had this since i was about 10 years old? what could it be?

Histamine release: Often the extremes of temperature will cause the release of histamine from cells, called mast cells in your body. Use of tepid water or a non drowsy antihistamine may help. It would be interesting to know if showers cause you the same problem. A trial of an oatmeal product in your bath may give some relief.

Dry skin: Dry skin comes to mind as most common reason. After warm bath, the capillary blood vessels of the skin dilate or open larger and this can sometimes cause a slight histamine reaction in the skin that can lead to itching. Trial of moisturizer right after you get out might help if dry skin. If not, try over the counter anti allergy med several hours before bath and see if it helps. Best of luck.