I'd like to further add to this - it could also (but probably isn't) something like ulcerative colitis or Crohn's. Often you'll go off food and feel sick/have minor abdominal pain, but not in all cases, and if you're developing these conditions then symptoms can come in slowly.
Just go to your GP and tell them. They'll know what tests need to be done to rule this out. If it is one of the conditions I've mentioned then things will just get worse, and it can take quite a while to get a proper diagnosis for them already.