My body aches, both joints and muscles i think. not arthritis. can barely open doors now. what could this be? am 19 and 75 lbs, but always been small

See rheumatologist: If your primary care doctor has no answer/solution see an arthritis specialist or rheumatologist. Certain auto immune diseases can give you the symptoms you mention as well as viral diseases and many more uncommon illnesses.

Possible fibro: There are literally many things that can cause those symptoms, including fibromyalgia. If possible, have a rheumatologist evaluate the problem.