What could be causing all my teeth on the top and bottom left side hurt horribly?

Can be many things: Have a dentist evaluate why you are feeling pain on the side of your mouth. It could be anything from grinding and clenching to a periodontal condition or an abscessing tooth (or teeth). It may be necessary to have x-rays taken of the area to evaluate where the pain is coming from.

Various reasons: You could have fractured / broken teeth or deep decay, where the break or decay is close to or within the nerve center portion of the tooth / teeth. You may have severe periodontal disease (gum and bone loss/recession). Or a combination of many dental problems you need to see a dentist for a thorough evaluation and determination of your treatment options to take care of your dental problems.

A large cavity : Even one large cavity that allows bacteria to enter the tooth nerve could cause the whole side to hurt. It could also be symptoms of temporal/mandibular joint dysfunction or even a sinus infection. If you have a fever along with sinus congestion or a runny nose it is probably sinuses. I would see my dentist asap to rule out a dental infection.