I just went to the toilet and went to flush there was a tiny black bug crawling around i have also had symptoms of itching and swelling is this crabs?

Unlikely: Crabs or Pubic lice are VERY VEY tiny usually clear (no color) or white...That bug was probably an "incidental" creature from outside your bathroom! Hope this helps! Dr Z

Very doubtful: The pubic louse (Phthirus pubis) is about 1 to 2 mm in length & when magnified looks like a crab. It is wingless, tan to grayish-white, flattened dorsoventrally, has 3 clawed sets of legs & a head which is significantly smaller than the rest of the body. Pubic lice look different than head or body lice. They don’t have the capacity to move on smooth surfaces.