I got bit by a bug about a month ago and it turned into a huge bruise with a hard lump underneath it. what is it?

Abscess: It is important that you get this examined by your doctor. The lump may be the development of an abscess which may need incision and drainage and possibly antibiotics

Bite reaction?: Here is some general information about bites: https://patient.info/health/insect-bites-and-stings-leaflet You can get some local reactions but as Dr Afraz has mentioned is important to get checked out and urgently if unwell to exclude abscess. Please see a doctor for examination