Central Heating system losing pressure?

I have oil heating, a firebird system. When the heating is on there doesnt seem to be any drop in the pressure but when i turn the heating on for the water, and when i use the shower, the pressure does drop by about one-two bars each time. I have checked the attic for leaks. No sign. The radiators were bled in December. Please tell me its not a leak that i cant see.