Could be a bit of implantation bleeding. Was it a proper bleed or more like spotting? The only way to tell what's going on is to either wait it out and test again in a couple of days or have a couple of HCG tests. They don't do these on the NHS so you'd have to have them privately.
The fact that the tests are getting darker sounds like a good sign. Usually if it's a chemical pregnancy the lines get lighter. It's usually in the case of later miscarriages that the hormones stick around longer.