Pain that primarily affects the small joints can be due to rheumatoid arthritis, gout, lupus arthritis, osteoarthritis (unlikely at your age) or reactive arthritis. So, you need tests to rule these out one by one. Also, reactive arthritis could be due to allergic reactions. So, if you have rash or itch or food intolerance, then this possibility should be looked into. Please consult your PCP for primary examination followed by proper referral. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.