The past week i've been having sharp pains in my uppermiddle left pectoral muscle. it seems to only have the sharp pains when it is fully stretched

Injury to fibers: Sounds like an injury to a select group of muscle fibers in the pectoral muscle (which is very broad and has many different fibers that are used more or less depending on the specific action you are performing). Strain or tear would be treated with anti-inflammatories and rest. If symptoms persist or are associated with a mass or other finding, you should see a doctor. Hope this helps!