I was bitten in the uk by something and after a week it leaked brownish fluid with black bits when squeezed, i have a cavity. is that cause for worry?

Sounds like : You have an infection from the bite. You need to be checked by your physician before it gets worst . He may prescribe you oral antibiotics or do incision and drainage and culture it. It all depends on how bad the affected area looks then.

Not sure: Looks like an insect bite might have got infected. Unless examined by your doctor it is difficult to guess. Please see you doctor asap.