I believe i was bitten by a spider yesterday morning sometime, not too sure when, but first i noticed itching around my elbow then later in the evening i noticed the area was very hot so i looked and i had this huge red spot on the underside of my arm by

Skin : It sounds like you have the beginnings of cellulitus, and bacterial infection of the skin. Please see a doctor to be evaluated and placed on antibiotics before further spreading.

ER: if the redness has not subsided or is increasing go to an ER now as you may have been bitten by something that could cause tissue damage like a brown recluse, as the damage will be more severe the longer you wait to get treatment. The itching may indicate that you are allergic to whatever bit you. Ideally if you could find the spider and take it in to the ER this would be great.